Articles in the Storage category

  1. ZFS: Resilver Performance of Various RAID Schemas

    January 31, 2016

    When building your own DIY home NAS, it is important that you simulate and test drive failures before you put your important data on it. It makes sense to know what to do in case a drive needs to be replaced. I also recommend putting a substantial amount of data on your NAS and see how long a resilver takes just so you know what to expect.

    There are many reports of people building their own (ZFS-based) NAS who found out after a drive failure that resilvering would take days. If your chosen redundancy level for the VDEV would not protect against a second drive failure in the same VDEV (Mirror, RAID-Z) things may get scary. Especially because drives are quite bussy rebuilding data and the extra load on the remaining drives may increase the risk of a second failure.

    The chosen RAID level for your VDEV, has an impact on the resilver performance. You may chose to accept lower resilver performance in exchange for additional redundancy (RAID-Z2, RAID-Z3).

    I did wonder though how much those resilver times would differ between the various RAID levels. This is why I decided to run some tests to get some numbers.

    Test hardware

    I've used some test equipment running Debian Jessie + ZFS on Linux. The hardware is rather old and the CPU may have an impact on the results.

    CPU : Intel(R) Core(TM)2 Duo CPU     E7400  @ 2.80GHz
    RAM : 8 GB
    HBA : HighPoint RocketRaid 2340 (each drive in a jbod)
    Disk: Samsung Spinpoint F1 - 1 TB - 7200 RPM ( 12 x )
    

    Test method

    I've created a script that runs all tests automatically. This is how the script works:

    1. Create pool + vdev(s).
    2. Write data on pool ( XX % of pool capacity)
    3. Replace arbitrary drive with another one.
    4. Wait for resilver to complete.
    5. Log resilver duration o csv file.

    For each test, I fill the pool up to 25% with data before I measure resilver performance.

    Caveats

    The problem with the pool only being filled for 25% is that drives are fast at the start, but their performance deteriorates significantly as they fill up. This means that you cannot extrapolate the results and calculate resilver times for 50% or 75% pool usage, the numbers are likely worse than that.

    I should run the test again with 50% usage to see if we can demonstrate this effect.

    Beware that this test method is probably only suitable for DIY home NAS builds. Production file systems used within businesses may be way more fragmented and I've been told that this could slow down resilver times dramatically.

    Test result (lower is better)

    resilver graph

    The results can only be used to demonstrate the relative resilver performance differences of the various RAID levels and disk counts per VDEV.

    You should not expect the same performance results for your own NAS as the hardware probably differs significantly from my test setup.

    Observations

    I think the following observations can be made:

    1. Mirrors resilver the fastest even if the number of drives involved is increased.
    2. RAID-Z resilver performance is on-par with using mirrors when using 5 disks or less.
    3. RAID-Zx resilver performance deteriorates as the number of drives in a VDEV increases.

    I find it interesting that with smaller number of drives in a RAID-Z VDEV, rebuild performance is roughly on par with a mirror setup. If long rebuild times would scare you away from using RAID-Z, maybe it should not. There may be other reasons why you might shy away from RAID-Z, but this doesn't seem one of them.

    RAID-Z2 is often very popular amongst home NAS builders, as it offers a very nice balance between capacity and redundancy. Wider RAID-Z2 VDEVs are more space efficient, but it is also clear that resilver operations take longer. Because RAID-Z2 can tollerate the loss of two drives, I think longer resilver times seem like a reasonable tradeoff.

    It is clear that as you put more disks in a single RAID-Zx VDEV, rebuild times increase. This can be used as an argument to keep the number of drives per VDEV 'reasonable' or to switch to RAID-Z3.

    25% vs 50% pool usage

    To me, there's nothing special to see here. The resilver times are on average slightly worse than double the 25% resilver durations. As disks performance start to deteriorate as they fill up (inner tracks are shorter/slower) sequential performance drops. So this is why I would explain the results are slightly worse than perfect linear scaling.

    Final words

    I hope this benchmark is of interest to anyone and more importantly, you can run your own by using the aforementioned script. If you ever want to run your own benchmarks, expect the script to run for days. Leave a comment if you have questions or remarks about these test results or the way testing is done.

    Tagged as : ZFS
  2. The 'Hidden' Cost of Using ZFS for Your Home NAS

    January 02, 2016

    Many home NAS builders consider using ZFS for their file system. But there is a caveat with ZFS that people should be aware of.

    Although ZFS is free software, implementing ZFS is not free. The key issue is that expanding capacity with ZFS is more expensive compared to legacy RAID solutions.

    With ZFS, you either have to buy all storage you expect to need upfront, or you will be wasting a few hard drives on redundancy you don't need.

    This fact is often overlooked, but it's very important when you are planning your build.

    Other software RAID solutions like Linux MDADM lets you grow an existing RAID array with one disk at a time. This is also true for many hardware-based RAID solutions. This is ideal for home users because you can expand as you need.

    ZFS does not allow this!

    To understand why using ZFS may cost you extra money, we will dig a little bit into ZFS itself.

    Quick recap of ZFS

    zfs

    The schema above illustrates the architecture of ZFS. There are a few things you should take away from it.

    The main takeaway of this picture is that your ZFS pool and thus your file system is based on one or more VDEVs. And those VDEVs contain the actual hard drives.

    Fault-tolerance or redundancy is addressed within a VDEV. A VDEV is either a RAID-1 (mirror), RAID-5 (RAIDZ) or RAID-6 (RAIDZ2). It can even use tripple parity (RAID-Z3) but I doubt many of you will ever need that.

    So it's important to understand that a ZFS pool itself is not fault-tolerant. If you lose a single VDEV within a pool, you lose the whole pool. You lose the pool, all data is lost.

    You can't add hard drives to a VDEV

    Now it's very important to understand that you cannot add hard drives to a VDEV.

    This is the key limitation of ZFS as seen from the perspective of home NAS builders.

    To expand the storage capacity of your pool, you need to add extra VDEVs. And because each VDEV needs to take care of its own redundancy, you also need to buy extra drives for parity.

    I will quickly add that there is a way out: replace every hard drive in the VDEV, one by one, with a higher capacity hard drive. You will have to 'rebuild' or 'resilver' the VDEV after each replacement, but it will work, although it's a bit cumbersome and quite expensive.

    So back to the topic at hand: what does this limitation mean in real life? I'll give an example.

    Let's say you plan on building a small NAS with a capacity of four drives. Please don't create a three-drive RAID-Z thinking you can just add the fourth drive when you need to, because that's not possible.

    In this example, you would be better off buying the fourth drive upfront and create a four-drive RAID-Z. This is an example where you are forced to buy the extra space you don't need yet upfront because expanding is otherwise not possible.

    You could have expanded your pool with another VDEV consisting of a minimum of three drives (if you run RAID-Z) but the chassis has only room for one extra drive so that doesn't work.

    Planning your ZFS Build with the VDEV limitation in mind

    Many home NAS builders use RAID-6 (RAID-Z2) for their builds, because of the extra redundancy. This makes sense because a double drive failure is not something unheard of, especially during rebuilds where all drives are being taxed quite heavily for many hours.

    I personally would recommend running RAID-Z2 over RAID-Z1 if you go over five to six drives and to spend the extra money on the additional hard drive it requires. Actually, With RAID-Z2 or RAID-6, I think it's perfectly reasonable to run a single VDEV at home with up to 12 drives1.

    With RAID-Z2 however, the 'ZFS tax' is even more clearly visible. By having to add an additional VDEV, you will also lose two drives due to parity overhead.

    zfs2

    Please note that the 'yellow' drives mark the parity/redundancy overhead. It does not mark where parity data lives (it's striped across all drives).

    Let's illustrate the above picture with an example. Your NAS chassis can hold a maximum of twelve drives. You start out with six drives in a RAID-Z2. At some point you want to expand. The cheapest option is to expand with another RAID-Z2 consisting of four drives (minimum size of a RAID-Z2 VDEV).

    With a cost of $150 per hard drive3, expanding the capacity of your pool will cost you $600 instead of $150 (single drive) and $300 dollar of the $600 (50%) is wasted on redundancy you don't really need.

    Furthermore, you can no longer expand your pool, so the remaining two drive slots are 'wasted'2. You end up with a maximum of ten drives.

    In this example, to make use of the drive capacity of your NAS chassis, you should expand with another six hard drives. That would cost you $900 and $300 of that $900 (33%) is wasted on redundancy. This is illustrated above.

    Storage-wise it's more efficient to expand with six drives instead of four. But it will cost you another $300 to expand, paying for storage you may not immediately need.

    But both options aren't that efficient. Because you end up using four drives for parity where two would - in my view - be sufficient.

    So, if you want to get the most capacity out of that chassis, and the most space per dollar, your only option is to buy all twelve drives upfront and create a single RAID-Z2 consisting of twelve drives.

    zfs1

    Buying all drives upfront is expensive and you may only benefit from that extra space years down the road.

    Summary

    So I hope this example clearly illustrates the issue at hand. With ZFS, you either need to buy all storage upfront or you will lose hard drives to redundancy you don't need, reducing the maximum storage capacity of your NAS.

    You have to decide what your needs are. ZFS is an awesome file system that offers you way better data integrity protection than other file system + RAID solution combination.

    But implementing ZFS has a certain 'cost'. You must decide if ZFS is worth it for you.

    Addressing some feedback

    I found out that my article was discussed on a vodcast of BSDNOW.

    This article also got some attention on hacker news.

    To me, some of the feedback is not 'wrong' but feels rather disingenuous or not relevant for the intended audience of this article. I have provided the links so you can make up your own mind.

    This article has a particular user group in mind so you really should think about how much their needs align with yours.

    You are steering people away from ZFS

    No I don't and this is not my intention. I run ZFS myself on two servers. I do feel that sometimes the downsides of ZFS are wiped under the rug and we should be very open and clear about them towards people seeking advice.

    Use mirrors not RAID-Z(2/3)!

    Doesn't make much sense to me for home NAS builders.

    Using mirrors is wasting space

    Advising people to use mirrors instead of RAID-Z(2/3) I do find a little bit disingenuous. Because you are throwing away 50% of your disk capacity. With RAIDZ you 'lose' 33% for three drives, 25% for four drives. If we look at RAIDZ2, we would 'lose' 33% for six drives, 25% for eight drives and only 20% for ten drives.

    In the end, you are waisting multiple drives worth of storage capacity depending on the number of drives in your pool.

    Adding mirrors with larger drives

    As time goes by, larger disks become cheaper. So it could make sense to expand your pool with mirrors based on bigger drives than the original drives you started out on. The size of your pool would increase. However, it's still only 50% space efficient.

    Random I/O performance is better

    Using mirrors is running RAID 10. Yes you can expand your pool with two drives at a time, and you gain better random I/O performance. However, the large majority of home NAS builders don't care about random I/O performance. You just care if you can saturate gigabit and have one big pool of storage. In that case, you don't need the random IOPs.

    If you run some VMs from your storage that require high storage performance, it's an entirely different matter. But I expect that most DIY NAS builders just want some storage to put a ton of data on and nothing more.

    RAIDZ2 is more reliable than using mirrors

    The redundancy of RAIDZ2 beats using mirrors. (If during a rebuild the surviving member of a mirror fails (the one disk in the pool that is taxed the most during rebuild) you lose your pool. With RAIDZ2 any second drive can fail and you are still OK.

    There is only one 'upside' regarding mirrors that is discussed in the next section.

    Mirror rebuild times are better

    The only upside of using mirrors is that in the event a disk has failed and the new disk is being 'resilvered' it is reported that those rebuilds tend to be faster than if you use RAID-Z(2/3). I think this is no different from legacy RAID, where the main difference with ZFS is that ZFS only rebuilds actual data, not the entire disk.

    ZFS rebuilds are faster

    This is indeed a benefit of ZFS. The question is how relevant it is for you.

    ZFS only rebuilds data. Legacy RAID just rebuilds every 'bit' on a drive. The latter takes longer than the former. So with legacy RAID, rebuild times depend on the size of a single drive, not on the number of drives in the array, no matter how much data you have stored on your array.

    My old 18 TB server was based on a single twenty-drive RAID 6 using MDADM. It took 5 hours to rebuild a 1 TB drive. If you would have used 4 TB drives, it would have taken 20 hours if I'm allowed to extrapolate. With ZFS - if you would have been using only 50% of capacity - those rebuild times would have been half of this.

    Personally with RAID6 or with RAIDZ2, rebuild times aren't that a big of a deal as you can lose a second drive and still be safe.

    Just replace existing drives with bigger ones!

    I did briefly touch this option in the article above. I will address it again. The problem with this approach is twofold. First, you can't expand storage capacity as you need it. You need to replace all existing drives with larger ones.

    The procedure itself is also a bit cumbersome and time intensive. You need to replace each drive one by one. And every time, you need to 'resilver' your VDEV. Only when all drives have been replaced you will be able to grow the size of your pool.

    If you are OK with this approach - and people have used it - it is a way to work around the 'ZFS-tax'.

    Not using ZFS is putting your data at great risk!

    The BSDNOW podcasts seems to agree with me that if you want true data safety, this 'ZFS-tax' is just the price you have to pay. Either you go with mirrors or you accept the extra parity redundancy.

    It is not my goal to steer you away from ZFS. The above is true. ZFS offers something no other (stable) file system currently offers to home NAS builders. But at a cost.

    The thing is that I find it perfectly reasonable for home NAS users to just buy a Synology, QNAP or some ready-made NAS from another quality brand. That's what the majority of people do and I think it's a reasonable option. I don't think you are taking crazy risks if you would do so.

    If you do build your own home NAS, it's reasonable to accept the 'risk' of using Windows with storage spaces or hardware RAID. Or using Linux with MDADM or hardware RAID. I would say: ZFS is clearly technically the better option, but those 'legacy' options are not so bad that you are taking unreasonable risks with your data.

    So using ZFS is the better option, it's up to you and your particular needs and circumstances to decide if using ZFS is worth it for you.


    1. For my own 71 TB storage NAS I decided at that time to run with an eighteen-disk VDEV plus a six-disk VDEV. Not standard, but I decided that I accept the risk.  

    2. Expanding with a VDEV consisting of a mirrored pair is technically possible but it breaks the RAID-Z2 redundancy. It doesn't make much sense to me. 

    3. Just an example for illustration purposes.  

    Tagged as : ZFS
  3. ZFS Performance on HP Proliant Microserver Gen8 G1610T

    August 14, 2015

    I think the HP Proliant Microserver Gen8 is a very interesting little box if you want to build your own ZFS-based NAS. The benchmarks I've performed seem to confirm this.

    The Microserver Gen8 has nice features such as:

    • iLO (KVM over IP with dedicated network interface)
    • support for ECC memory
    • 2 x Gigabit network ports
    • Free PCIe slot (half-height)
    • Small footprint
    • Fairly silent
    • good build quality

    The Microserver Gen8 can be a better solution than the offerings of - for example - Synology or QNAP because you can create a more reliable system based on ECC-memory and ZFS.

    gen8

    Please note that the G1610T version of the Microserver Gen8 does not ship with a DVD/CD drive as depicted in the image above.

    The Gen8 can be found fairly cheap on the European market at around 240 Euro including taxes and if you put in an extra 8 GB of memory on top of the 2 GB installed you have a total of 10 GB, which is more than enough to support ZFS.

    The Gen8 has room for 4 x 3.5" hard drives so with todays large disk sizes you can pack quite a bit of storage inside this compact machine.

    gen82

    Netto storage capacity:

    This table gives you a quick overview of the netto storage capacity you would get depending on the chosen drive size and redundancy.

    Drive sizeRAIDZRAIDZ2 or Mirror
    3 TB 9 TB 6 TB
    4 TB12 TB 8 TB
    6 TB18 TB12 TB
    8 TB24 TB16 TB

    Boot device

    If you want to use all four drive slots for storage, you need to boot this machine from either the fifth internal SATA port, the internal USB 2.0 port or the microSD card slot.

    The fifth SATA port is not bootable if you disable the on-board RAID controller and run in pure AHCI mode. This mode is probably the best mode for ZFS as there seems to be no RAID controller firmware active between the disks and ZFS. However, only the four 3.5" drive bays are bootable.

    The fifth SATA port is bootable if you configure SATA to operate in Legacy mode. This is not recommended as you lose the benefits of AHCI such as hot-swap of disks and there are probably also performance penalties.

    The fifth SATA port is also bootable if you enable the on-board RAID controller, but do not configure any RAID arrays with the drives you plan to use with ZFS (Thanks Mikko Rytilahti). You do need to put the boot drive in a RAID volume in order to be able to boot from the fifth SATA port.

    The unconfigured drives will just be passed as AHCI devices to the OS and thus can be used in your ZFS array. The big question here is what happens if you encounter read errors or other drive problems that ZFS could handle, but would be a reason for the RAID controller to kick a drive off the SATA bus. I have no information on that.

    I myself used an old 2.5" hard drive with a SATA-to-USB converter which I stuck in the case (use double-sided tape or velcro to mount it to the PSU). Booting from USB stick is also an option, although a regular 2.5" hard drive or SSD is probably more reliable (flash wear) and faster.

    Boot performance

    The Microserver Gen8 takes about 1 minute and 50 seconds just to pass the BIOS boot process and start booting the operating system (you will hear a beep).

    Test method and equipment

    I'm running Debian Jessie with the latest stable ZFS-on-Linux 0.6.4. Please note that reportedly FreeNAS also runs perfectly fine on this box.

    I had to run my tests with the disk I had available:

    root@debian:~# show disk -sm
    -----------------------------------
    | Dev | Model              | GB   |   
    -----------------------------------
    | sda | SAMSUNG HD103UJ    | 1000 |   
    | sdb | ST2000DM001-1CH164 | 2000 |   
    | sdc | ST2000DM001-1ER164 | 2000 |   
    | sdd | SAMSUNG HM250HI    | 250  |   
    | sde | ST2000DM001-1ER164 | 2000 |   
    -----------------------------------
    

    The 250 GB is a portable disk connected to the internal USB port. It is used as the OS boot device. The other disks, 1 x 1 TB and 3 x 2 TB are put together in a single RAIDZ pool, which results in 3 TB of storage.

    Tests with 4-disk RAIDZ VDEV

    root@debian:~# zfs list
    NAME       USED  AVAIL  REFER  MOUNTPOINT
    testpool  48.8G  2.54T  48.8G  /testpool
    root@debian:~# zpool status
      pool: testpool
     state: ONLINE
      scan: none requested
    config:
    
        NAME                        STATE     READ WRITE CKSUM
        testpool                    ONLINE       0     0     0
          raidz1-0                  ONLINE       0     0     0
            wwn-0x50000f0008064806  ONLINE       0     0     0
            wwn-0x5000c5006518af8f  ONLINE       0     0     0
            wwn-0x5000c5007cebaf42  ONLINE       0     0     0
            wwn-0x5000c5007ceba5a5  ONLINE       0     0     0
    
    errors: No known data errors
    

    Because a NAS will face data transfers that are sequential in nature, I've done some tests with 'dd' to measure this performance.

    Read performance:

    root@debian:~# dd if=/testpool/test.bin of=/dev/null bs=1M
    50000+0 records in 50000+0 records out 52428800000 bytes (52 GB) copied, 162.429 s, 323 MB/s

    Write performance:

    root@debian:~# dd if=/dev/zero of=/testpool/test.bin bs=1M count=50000 conv=sync 50000+0 records in 50000+0 records out 52428800000 bytes (52 GB) copied, 169.572 s, 309 MB/s

    Test with 3-disk RAIDZ VDEV

    After the previous test I wondered what would happen if I would exclude the older 1 TB disk and create a pool with just the 3 x 2 TB drives. This is the result:

    Read performance:

    root@debian:~# dd if=/testpool/test.bin of=/dev/null bs=1M conv=sync 50000+0 records in 50000+0 records out 52428800000 bytes (52 GB) copied, 149.509 s, 351 MB/s

    Write performance:

    root@debian:~# dd if=/dev/zero of=/testpool/test.bin bs=1M count=50000 conv=sync 50000+0 records in 50000+0 records out 52428800000 bytes (52 GB) copied, 144.832 s, 362 MB/s

    The performance is clearly better even there's one disk less in the VDEV. I would have liked to test with an additional 2 TB drive what kind of performance would be achieved with four drives but I only have three.

    The result does show that the pool is more than capable of sustaining gigabit network transfer speeds.

    This is confirmed when performing the actual network file transfers. In the example below, I simulate a copy of a 50 GB test file from the Gen8 towards a test system using NFS. Tests are performed using the 3-disk pool.

    NFS read performance:

    root@nano:~# dd if=/mnt/server/test2.bin of=/dev/null bs=1M
    50000+0 records in
    50000+0 records out
    52428800000 bytes (52 GB) copied, 443.085 s, 118 MB/s
    

    NFS write performance:

    root@nano:~# dd if=/dev/zero of=/mnt/server/test2.bin bs=1M count=50000 conv=sync 
    50000+0 records in
    50000+0 records out
    52428800000 bytes (52 GB) copied, 453.233 s, 116 MB/s
    

    I think these results are excellent. Tests with the 'cp' command give the same results.

    I've also done some test with the SMB/CIFS protocol. I've used a second Linux box as a CIFS client to connect to the Gen8.

    CIFS read performance:

    root@nano:~# dd if=/mnt/test/test.bin of=/dev/null bs=1M
    50000+0 records in
    50000+0 records out
    52428800000 bytes (52 GB) copied, 527.778 s, 99.3 MB/s
    

    CIFS write performance:

    root@nano:~# dd if=/dev/zero of=/mnt/test/test3.bin bs=1M count=50000 conv=sync
    50000+0 records in
    50000+0 records out
    52428800000 bytes (52 GB) copied, 448.677 s, 117 MB/s
    

    Hot-swap support

    Although it's even printed on the hard drive caddies that hot-swap is not supported, it does seem to work perfectly fine if you run the SATA controller in AHCI mode.

    Fifth SATA port for SSD SLOG/L2ARC?

    If you buy a converter cable that converts a floppy power connector to a SATA power connector, you could install an SSD. This SSD can then be used as a dedicated SLOG device and/or L2ARC cache if you have a need for this.

    RAIDZ, is that OK?

    If you want maximum storage capacity with redundancy RAIDZ is the only option. RAID6 or two mirrored VDEVs is more reliable, but will reduce available storage space by a third.

    The main risk of RAIDZ is a double-drive failure. As with larger drive sizes, a resilver of a VDEV will take quite some time. It could take more than a day before the pool is resilvered, during which you run without redundancy.

    With the low number of drives in the VDEV the risk of a second drive failure may be low enough to be acceptable. That's up to you.

    Noise levels

    In the past, there have been reports about the Gen8 making tons of noise because the rear chasis fan spins at a high RPM if the RAID card is set to AHCI mode.

    I myself have not encountered this problem. The machine is almost silent.

    Power consumption

    With drives spinning: 50-55 Watt. With drives standby: 30-35 Watt.

    Conclusion

    I think my benchmarks show that the Microserver Gen8 could be an interesting platform if you want to create your own ZFS-based NAS.

    Please note that it is likely that since the Gen9 server platform is already out for some time, HP may release a Gen9 version of the microserver in the near future. However as of August 2015, there is no information on this yet and it is not clear if a successor is going to be released.

    Tagged as : ZFS microserver

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