Adding SSD for cache - two ZFS pools

Discussion in 'Proxmox VE: Installation and configuration' started by nightcom, May 17, 2019 at 19:27.

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  1. nightcom

    nightcom New Member

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    Hi all,
    I would like to improve performance in my second pool 3x1TB HDD RAIDz1,I want to lower I/O delay its not high but I was reading also that there shouldn't be any I/O delay, (first pool where is Proxmox I have 2xSSD in RAIDz) by adding SSD drive as cache. I found already on Google some information's how to do it
    after creating two partitions on SSD (same size?)

    zpool add rpool cache /dev/sdf1
    zpool add rpool log /dev/sdf2
    zpool status

    and now I can't find right answer and that's why I came here again for advice :)
    My questions are
    1. If I want just for second pool should I write name of second pool instead "rpool" (zpool add secondpoolname cache....)?
    2. What will happend when SSD with cache will die? Just performance will drop or some data can be lost also?
    3. 2xSSD in Raidz that I'm using for Proxmox, is it good idea to create cache on same SSD in this Raidz and Proxmox on it?
     
    #1 nightcom, May 17, 2019 at 19:27
    Last edited by a moderator: May 18, 2019 at 08:19
  2. nightcom

    nightcom New Member

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  3. wolfgang

    wolfgang Proxmox Staff Member
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    Hi,

    Generall, it makes no sense to add an SSD as cache (ZIL) device to an SSD Pool.
    A Cache will only improve the speed if the cache device is faster then the Pool.
    Also, the ARC as to be used with care, because it is not for free and will need about 300KB per block in memory.

    For the L2ARC it makes no difference.
    And for the ZIL it depends.
    1.) only the ZIL is failing and the Server stays running nothing happened because the Data are in Memory.
    2.) the Server crashes and the ZIL is dead, you will lose all the data from the not committed Transaction Group. But the FS stays consistent.
    A Raidz aka raid5 makes no sense for two disks.
    The cache device has to be a dedicated device if you like to see any performance increase
     
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