Buying a dualport SAS SSD for ZIL and some questions about sector size

Discussion in 'SSDs & Data Storage' started by levak, May 6, 2015.

  1. levak

    levak Limp Gawd

    Mar 27, 2011

    I already have a topic on servethehome, but I said I'll ask here as well and maby get some more info.

    So, we are rebuilding our OmniOS SAN and will build a HA cluster with the help of RSF-1 software. For us to be able to connect out JBODs to 2 controllers, we need a dual-port drives. No problem with hard drives, since most of the enterprise SAS hard drives are dual ported and most of them have same-same performance.

    I'm currently stuck as selecting the right SSD for SLOG (according to zilstat, we need SLOG, since we have a lot of sync writes over iSCSI).

    Currently my selection the following:
    - HGST s842
    - HGST SSD800MM
    - Optimus Extreme™ SSDs
    - Optimus Ultra/Ascend (same price as extreme, but more space and lower DWPD, so extreme is probably Ascend with a lot of overprovisioning?)
    - Samsung SM1625 (can't find them anywhere but on their web page)
    - Seagate S1200 SAS SSD (no info on DWPD)
    - Toshiba PX02SSF010 ( how about these toshiba's? Are they any good?)

    What do you use?
    What do you recommend?

    A few questions about sector size:
    - should I go with 512b or 4k sector size hard drives?
    - should I go with 512b or 4k sector size SSD SLOG drives?
    - can I have 4k hard drives and 512b SSD SLOG?
    - for hard drives, if I can choose 4k or 512e, what to pick?
    - how much performance hit is there with 512e SSD drives, I know that with spindles, there is at least one plate turn, but since SSD don't have that....?

    Thanks, Matej
  2. patrickdk

    patrickdk Gawd

    Jan 3, 2012
    Surprised your not thinking about a zeusram option.

    For iscsi usage, it wont matter if you use 512b, 512e, or 4k for the disks. As your blocksize will be atleast 4k on the filesystem ontop of it. You should figure out what blocksize you will use for your zvol's for iscsi though, before you can figure this out. If you go with 8k or something, not a real issue, but using 512e or 4k will waste a lot of metadata space.

    As far as I know, slog only supports 512b, but might be wrong, I know there is work to make l2arc support 4k, and I would assume, but don't know for sure if that also is the same for slog.

    It shouldn't matter about 4k or 512e, they will be acted on the same, assuming the os you use, doesn't have an issue with 4k disks.

    512e ssd's performance hit depends on many things. How much writing is done, how random that writing is, how much cache ram the ssd has on it. For zfs and slog, the writes will always be streamed, making the most optimial write case for ssd, so it should not cause any performance impact. But then, all ssd's are either 4k, 8k, or even 16k

    I'm not sure what you mean by hdd have one spindle slowdown. This is true of all hdd, 512b, 512e, 4k. It's just the drive latency, ssd's have the same thing. and the slowdown due to 512e vs 512b/4k is the same for hdd and ssd.
  3. levak

    levak Limp Gawd

    Mar 27, 2011
    A little late reply from my side.

    Thanks for info. I did some more reading in the mean time and figured the same things you pointed out...

    I won't go with ZeusRAM, because it's too expensive for us. A good SAS SSD will have to do. Probably something like HGST SSD800MH.B or Seagate S1200.

    Thanks, Matej