# [OmniOS-discuss] Pliant/Sandisk SSD ZIL

Richard Elling richard.elling at richardelling.com
Tue Feb 18 00:31:42 UTC 2014

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    On Feb 17, 2014, at 2:48 PM, Derek Yarnell <derek at umiacs.umd.edu> wrote:

    > Hi,
    >
    > So we bought a new Dell R720xd with 2 Dell SLC SSDs which were shipped
    > as a Pliant-LB206S-D323-186.31GB via format.

    Pliant SSDs (note: Pliant was purchased by Sandisk in 2011) are optimized for
    lots of concurrent I/O operations. This is not the kind of workload generated by
    the ZIL, which is more contiguous, single thread-like.

    >  The bulk of the storage is
    > just a 6+2 RaidZ2 with 3TB disks.  When doing a simple untar (40MB/2000
    > files) over NFS (synchronous IO) having the mirrored slog of Pliant SSDs
    > seems to offer horrendous performance.  On the order of 18-20 OPs per
    > second (via zpool iostat -v 1).

    Never ever use zpool iostat to measure application performance. zpool iostat
    measures workload to the vdevs, showing back-end operations to disk. As such
    there is no correlation to client-side operations of any sort, especially writes and
    metadata updates. You'll need to go up the stack and see what it is doing. For NFS
    I highly recommend nfssvrtop. To see the response time of the Pliant, use "iostat -x"
    or, as I prefer, "iostat -zxCn"

    Note: response time (measured by iostat -x as a variant of "svc_t") is the critical
    measurement for NFS workloads. Bandwidth is almost meaningless in analyzing
    NFS.

    >  We have a number of other ZFS builds
    > with ZeusRAM slogs and they work very well and while the Pliant may not
    > be as good a choice it shouldn't be as bad as this.

    It can be as bad as that.

    > # zfs get sync,dedup,logbias,compression,checksum zpool0
    > NAME    PROPERTY     VALUE          SOURCE
    > zpool0  sync         standard       default
    > zpool0  dedup        off            default
    > zpool0  logbias      latency        default
    > zpool0  compression  off            default
    > zpool0  checksum     on             default
    >
    > Adding a ramdisk slog device makes everything perform as expected.

    yep.  A ZeusRAM will also help.
     -- richard

    > Setting sync=disabled of course makes things all better.
    >
    > # zpool status zpool0
    >  pool: zpool0
    > state: ONLINE
    >  scan: none requested
    > config:
    >
    >        NAME                       STATE     READ WRITE CKSUM
    >        zpool0                     ONLINE       0     0     0
    >          raidz2-0                 ONLINE       0     0     0
    >            c1t5000C50057EC4B73d0  ONLINE       0     0     0
    >            c1t5000C50057EC6EABd0  ONLINE       0     0     0
    >            c1t5000C50057EC7CB7d0  ONLINE       0     0     0
    >            c1t5000C50057EC8F23d0  ONLINE       0     0     0
    >            c1t5000C50057EC998Bd0  ONLINE       0     0     0
    >            c1t5000C50057ECB1BBd0  ONLINE       0     0     0
    >            c1t5000C50057ECFEE7d0  ONLINE       0     0     0
    >            c1t5000C50057ED03F7d0  ONLINE       0     0     0
    >        logs
    >          mirror-1                 ONLINE       0     0     0
    >            c3t5001E820026DA2CAd0  ONLINE       0     0     0
    >            c3t5001E820026DA45Ad0  ONLINE       0     0     0
    >
    > errors: No known data errors
    >
    >
    > --
    > Derek T. Yarnell
    > University of Maryland
    > Institute for Advanced Computer Studies
    > _______________________________________________
    > OmniOS-discuss mailing list
    > OmniOS-discuss at lists.omniti.com
    > http://lists.omniti.com/mailman/listinfo/omnios-discuss

    --

    Richard.Elling at RichardElling.com
    +1-760-896-4422

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