Date: Sun, 16 Oct 2011 21:02:06 +0300 From: "Luchesar V. ILIEV" <luchesar.iliev@gmail.com> To: Daniel Kalchev <daniel@digsys.bg>, Patrick Donnelly <batrick@batbytes.com> Cc: freebsd-fs@freebsd.org Subject: Re: [ZFS] Using SSD with partitions Message-ID: <4E9B1C1E.7090804@gmail.com> In-Reply-To: <169E82FD-3B61-4CAB-B067-D380D69CDED5@digsys.bg> References: <CACh33Fpz=uAp8h0Bjsi1Be=ob_94jXtN51mAHvGPkReY5MpTcg@mail.gmail.com> <4E9AE725.4040001@gmail.com> <169E82FD-3B61-4CAB-B067-D380D69CDED5@digsys.bg>
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On 16/10/2011 19:17, Daniel Kalchev wrote: > > On Oct 16, 2011, at 17:16 , Luchesar V. ILIEV wrote: > >> 6. If, OTOH, you're running a reasonably recent -STABLE (8 or 9), >> then your zpool version is likely 28 (thanks, pjd@), which means >> ZIL is not that scary, but you might still lose some data. Even an >> unexpected power failure might cause trouble, unless the SSD is >> designed to handle it gracefully (this typically involves some sort >> of capacitor). > > Just for the record: even without ZIL, you will most definitely lose > data at power outage. In most cases, this will not damage the ZFS > filesystem, but data will be lost. There is nothing that can prevent > this. > > Therefore, with ZFS v28, adding ZIL does not introduce any more risk > to your data. I might be wrong in my interpretation, but from what I remember, when the power goes down, an unprotected SSD is likely to lose _more_ data than simply its write buffers -- that's quite unlike a hard-drive. So much, in fact, that the whole ZIL might become corrupted (and that's potentially way more data than any device cache). _If_ that's true, then isn't an array of only "conventional" HDDs, where the ZIL is interleaved with the zpool itself, at least a bit safer from power failures? Again, if we are taking the cheaper SSDs into account. > One thing to have in mind is ZIL will help only under certain > workloads, sequential write is not one of these. It helps most with > database-type loads and sync writes like an NFS server that is > written heavily. Freddie have good advice on determining if it will > help. > > L2ARC on the other hand may help enormously, especially if the spool > is big. Workstation-class motherboards until recently were topped at > 8GB RAM and ZFS is happy with as much RAM as you can offer. Adding > L2ARC may provide more headroom. Benefits of course depend on the > workload. Neither L2ARC or ZIL provide magical benefits. Which is yet another reason to go for more RAM, as it tends to be quite magic-yielding. Just kidding here, but, seriously, if Patrick has room for some RAM upgrade, I think he should consider this, at least for performance (a boot and OS drive, obviously, are another matter). Cheers, Luchesar
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