Date: Wed, 27 Mar 2013 10:14:57 +0000 From: Tom Evans <tevans.uk@googlemail.com> To: Zeus Panchenko <zeus@ibs.dn.ua> Cc: freebsd-fs@freebsd.org Subject: Re: RAM amount recommendations for ZFS pools with ZIL and L2ARC on SSD Message-ID: <CAFHbX1LyHoG3o-Tkdogf9gsdwBdCHepCr%2Bo7ptdYy%2Biz9W1G1A@mail.gmail.com> In-Reply-To: <20130322172657.95912@relay.ibs.dn.ua> References: <20130322172657.95912@relay.ibs.dn.ua>
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On Fri, Mar 22, 2013 at 2:26 PM, Zeus Panchenko <zeus@ibs.dn.ua> wrote: > hi all, > > while discovering the subj, I found recommendations at: > http://doc.freenas.org/index.php/Hardware_Recommendations#RAM > > - --------------------------------------------------------------------- > ... a general rule of thumb is 1 GB of RAM for every 1TB of storage ... > If you plan to use ZFS deduplication, a general rule of thumb is 5 GB > RAM per TB of storage to be deduplicated. > - --------------------------------------------------------------------- > > > so, are these recommendations correct for ZFS pools with ZIL and L2ARC > on SSD configurations? > > are there corellations between "RAM amount" and "with/without ZIL, L2ARC > on separate devices pool configuration" ? > Well, they wouldn't hurt. Personally, I've used much less than that - I have a 12 x 1.5 TB server with 8 GB RAM, which is only 0.5GB/TB. It's replacement will have significantly more, but only because RAM is cheap(er). Dedupe is a special beast, to get decent performance the whole dedupe table must fit into memory, Oracle have a sizing guide: http://www.oracle.com/technetwork/articles/servers-storage-admin/o11-113-size-zfs-dedup-1354231.html If you don't have a ZIL, a portion of your pool is used instead as ZIL. RAM is never used for ZIL. Remember that the ZIL is only for synchronous writes, which most writes are not. RAM is mainly used for the ARC cache, and RAM is a lot faster than an SSD, so the more RAM used for the ARC cache the better. Adding an SSD L2ARC in addition will increase the amount of things that can be kept cached in a (relatively) slow cache. If your problem is that you need that data in a faster cache, then you need more RAM. I think I am right in saying that only things that have been read can be stored in ARC, so how effective it is would depend upon your workload - how much read things do you need in the cache? Cheers Tom
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