ZFS Features


ZFS includes two exciting features that dramatically improve the performance of read operations. I’m talking about ARC and L2ARC. ARC stands for adaptive replacement cache. ARC is a very fast block level cache located in the server’s memory (RAM). The amount of ARC available in a server is usually all of the memory except for 1GB.

For example, our ZFS server with 12GB of RAM has 11GB dedicated to ARC, which means our ZFS server will be able to cache 11GB of the most accessed data. Any read requests for data in the cache can be served directly from the ARC memory cache instead of hitting the much slower hard drives. This creates a noticeable performance boost for data that is accessed frequently.

As a general rule, you want to install as much RAM into the server as you can to make the ARC as big as possible. At some point adding more memory becomes cost prohibitive, which is where the L2ARC becomes important. The L2ARC is the second level adaptive replacement cache. The L2ARC is often called “cache drives” in the ZFS systems.

These cache drives are physically MLC style SSD drives. These SSD drives are slower than system memory, but still much faster than hard drives. More importantly, the SSD drives are much cheaper than system memory. Most people compare the price of SSD drives with the price of hard drives, and this makes SSD drives seem expensive. Compared to system memory, MLC SSD drives are actually very inexpensive.

When cache drives are present in the ZFS pool, the cache drives will cache frequently accessed data that did not fit in ARC. When read requests come into the system, ZFS will attempt to serve those requests from the ARC. If the data is not in the ARC, ZFS will attempt to serve the requests from the L2ARC. Hard drives are only accessed when data does not exist in either the ARC or L2ARC. This means the hard drives receive far fewer requests, which is awesome given the fact that the hard drives are easily the slowest devices in the overall storage solution.

In our ZFS project, we added a pair of 160GB Intel X25-M MLC SSD drives for a total of 320GB of L2ARC. Between our ARC of 11GB and our L2ARC of 320GB, our ZFS solution can cache over 300GB of the most frequently accessed data! This hybrid solution offers considerably better performance for read requests because it reduces the number of accesses to the large, slow hard drives.


Things to Keep in Mind

There are a few things to remember. The cache drives don’t get mirrored. When you add cache drives, you cannot set them up as mirrored, but there is no need to since the content is already mirrored on the hard drives. The cache drives are just a cheap alternative to RAM for caching frequently access content.

Another thing to remember is you still need to use SLC SSD drives for the ZIL drives.  ZIL stands for  "ZFS Intent Log", and acts as an intermediary for write caching.  Not having ZIL drives severely slows down write access.  By adding the ZIL drives you significantly increase write speeds.  This is still not as fast as a RAM based write cache on a RAID card, but it is much better than not having anything. Solaris ZFS Best Practices For Log Devices The SLC SSD drives used for ZIL drives dramatically improve the performance of write actions. The MLC SSD drives used as cache drives are used to improve read performance.

It is also important to remember that the L2ARC will require some memory to operate.  A portion of the ARC will be used to index and manage the content located in the L2ARC.  A general rule of thumb is that 1-2GB of ARC will be used for every 100GB of L2ARC.  With a 300GB L2ARC, we will give up 3-6GB of ARC.  This will leave us with 5-8GB of ARC memory to use to cache the most frequently accessed files.


Effective Caching to Virtualized Environments

At this point, you are probably wondering how effectively the two levels of caching will be able to cache the most frequently used data, especially when we are talking about 9TB of formatted RAID10 capacity. Will 11GB of ARC and 320GB L2ARC make a significant difference for overall performance? It will depend on what type of data is located on the storage array and how it is being accessed. If it contained 9TB of files that were all accessed in a completely random way, the caching would likely not be effective. However, we are planning to use the storage for virtual machine file systems and this will cache very effectively for our intended purpose.

When you plan to deploy hundreds of virtual machines, the first step is to build a base template that all of the virtual machines will start from. If you were planning to host a lot of Linux virtual machines, you would build the base template by installing Linux. When you get to the step where you would normally configure the server, you would shut off the virtual machine. At that point, you would have the base template ready. Each additional virtual machine would simply be chained off the base template. The virtualization technology will keep the changes specific to each virtual machine in its own child or differencing file.

When the virtualization solution is configured this way, the base template will be cached quite effectively in the ARC (main system memory). This means the main operating system files and cPanel files should deliver near RAM-disk performance levels. The L2ARC will be able to effectively cache the most frequently used content that is not shared by all of the virtual machines, such as the content of the files and folders in the most popular websites or MySQL databases. The least frequently accessed content will be pulled from the hard drives, but even that should show solid performance since it will be RAID10 across 18 drives and none of the frequently accessed read requests will need to burden the RAID10 volume since they are already served from ARC or L2ARC.


Testing the L2ARC

We thought it would be fun to actually test the L2ARC and build a chart of the performance as a function of time.  To test and graph usefulness of L2ARC, we set up an iSCSI share on the ZFS server and then ran Iometer from our test blade in our blade center.  We ran these tests over gigabit Ethernet.

 Iometer Test Details:

25GB working set

4k blocks

100% random

100% read

load 32 (constant)

four hour test

Every ten minutes during the test, we grabbed the “Last performance” values (IOPS, MB/sec) from Iometer and wrote them down to build a performance chart.  Our goal was to be able to graph the performance as a function of time so we could illustrate the usefulness of the L2ARC.

We ran the same test using the Promise M610i (16 1TB WD RE3 drives in RAID10) box to get a comparison graph.  The Promise box is not a ZFS style solution and does not have any L2ARC style caching feature.  We expected the ZFS box to outperform the Promise box, and we expected the ZFS box to increase performance as a function of time because the L2ARC would become more populated the longer the test ran. 

The Promise box consistently delivered 2200 to 2300 IOPS every time we checked performance during the entire 4 hour test.  The ZFS box started by delivering 2532 IOPS at 10 minutes into the test and delivered 20873 IOPS by the end of the test.

Here is the chart of the ZFS box performance results:

L2ARC Performance


Initially, the two SAN boxes deliver similar performance, with the Promise box at 2200 IOPS and the ZFS box at 2500 IOPS.  The ZFS box with a L2ARC is able to magnify its performance by a factor of ten once the L2ARC is completely populated! 

Notice that ZFS limits how quickly the L2ARC is populated to reduce wear on the cache drives.  It takes a few hours to populate the L2ARC and achieve maximum performance.  That seems like a long time when running benchmarks, but it is actually a very short period of time in the life cycle of a typical SAN box.

What is ZFS? Other Cool ZFS Features


View All Comments

  • Mattbreitbach - Tuesday, October 05, 2010 - link

    Indeed you can, which is one of the most exciting parts about using software based storage appliances. Nexenta really excels in this area, offering iSCSI, NFS, SMB, and WebDAV with simple mouse clicks. Reply
  • MGSsancho - Tuesday, October 05, 2010 - link

    or a single command! Reply
  • FransUrbo - Wednesday, January 11, 2012 - link

    Would be really nice to see how ZoL compares. It's in no way optimized yet (current work is on getting the core functionality stable - which IMHO it is) so it would have no chanse against OpenSolaris or Nexenta, but hopfully it's comparative to the Promise rack.

  • gfg - Tuesday, October 05, 2010 - link

    NAS is extremely cost effective in a data center if a large majority of NFS/CIFS users are more interested in capacity, not performance. NDMP can be very efficent for backups, and the snapshots/multi-protocol aspects of NAS systems are fairly easy to manage. Some of the larger Vendor NAS systems can support 100+TB's per NAS fairly effectively. Reply
  • bhigh - Wednesday, October 06, 2010 - link

    Actually, OpenSolaris and Nexenta can act as a SAN device using COMSTAR. You can attach to them with iSCSI, FC, Infiniband, etc. and use any zvols as raw scsi targets. Reply
  • JGabriel - Wednesday, October 06, 2010 - link

    Also, "Testing and Benchmarking"?

    Doesn't that mean the same thing and isn't it redundant? See what I did there?

  • Fritzr - Thursday, October 07, 2010 - link

    This is similar to the NAS<>SAN argument. They are used in a similar manner, but have very different purposes.

    Testing. You are checking to see if the item performance meets your need & looking for bugs or other problems including documentation and support.

    Benchmarking. You are running a series of test sets to measure the performance. Bugs & poor documentation/support may abort some of the measuring tools, but that simply goes into the report of what the benchmarks measured.

    Or in short:
    Test==does it work?
    Benchmark==What does it score on standard performance measures?
  • lwatcdr - Friday, October 08, 2010 - link

    I am no networking expert so please bear with me.
    What are the benfits of a SAN over local drivers and or a NAS?
    I would expect a NAS to have better performance since it would send less data over the wire than a SAN if they both had the same physical connection.
    A local drive/array I would expect to be faster than a SAN since it will not need to go through a network.
    Does it all come down to management? I can see the benefit of having your servers boot over the network and having all your drives in one system. If you set up the servers to boot over the network it would be really easy to replace a server.
    Am I missing something or are the gains all a matter of management?
  • JohanAnandtech - Sunday, October 10, 2010 - link

    A NAS has most of the time worse performance than a similar SAN since there is a file system layer on the storage side. A SAN only manages block and has thus less layers, and is more efficient.

    A local drive array is faster, but is less scalable and depending on the setup, it is harder to give a large read/write cache: you are limited by the amount of RAM your cache controller supports. In a software SAN you can use block based caches in the RAM of your storage server.

    Management advantages over Local drives are huge: for example you can plug a small ESXi/Linux flash drive which only contains the hypervisor/OS, and then boot everything else from a SAN. That means that chances are good that you never have to touch your server during its lifetime and handle all storage and VM needs centrally. Add to that high availability, flexibility to move VMs from one server to another and so on.
  • lwatcdr - Monday, October 11, 2010 - link

    I but that layer must be executed somewhere I thought that decrease in data sent over the physical wire would make up for the extra software cost on the server side.
    Besides you would still want a NAS even with a SAN for shared data. I am guessing that you could have a NAS served data from the SAN if you needed shared directories.
    I also assume that since most SAN are on a separate storage network that the SAN is mainly used to provide storage to servers and than the servers provide data to clients on the lan.
    The rest of it seems very logical to me in a large setup. I am guessing that if you have a really high performance data base server that one might use a DAS instead of SAN or dedicate a SAN server just to the database server.
    Thanks I am just trying to educate myself on SANs vs NAS vs DAS.
    Since I work at a small software development firm our sever setup is much simpler than the average Data center so I don't get to deal this level of hardware often.
    However I am thinking that maybe we should build a SAN and storage network just for our rack.

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