Date: Mon, 14 Jul 2014 15:40:02 +0300 From: Daniel Kalchev <daniel@digsys.bg> To: javocado <javocado@gmail.com> Cc: FreeBSD Filesystems <freebsd-fs@freebsd.org> Subject: Re: Using 2 SSD's to create a SLOG Message-ID: <53C3CFA2.9070309@digsys.bg> In-Reply-To: <CAP1HOmQuy7kZGg1kKW2=yct_3K50mnws7vQ369jNN3FpMczMvw@mail.gmail.com> References: <CAP1HOmRe5hEts6qA=0ZiNbMt%2B%2BDoyJO1f0fs=Wnx4ceC%2BygWeg@mail.gmail.com> <53BD0AFB.3000909@digsys.bg> <CAP1HOmQuy7kZGg1kKW2=yct_3K50mnws7vQ369jNN3FpMczMvw@mail.gmail.com>
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On 11.07.14 22:18, javocado wrote: > Thank you for your feedback. We understand that the devices are > mirrored and tolerant. > To further my understanding, confirm that I should be able to yank one > of the SLOG SSD drives and zfs won't care, and I can subsequently > replace it (issuing what command?) and zfs will then re-mirror it? And > if both drives are removed then only the sync data would be lost? But > if the drives are restored, or in the case of a power loss when the > server comes back up, the sync data on the SSD's will be processed > once zfs starts? > As the SLOG is only ever read at system boot time (actually, when importing the pool), there is no chance you lose data if both of your SLOG drives die *while the system is still running*. If this happens, ZFS will simply work as if no SLOG was configured. The only possible time you can lose data due to SLOG is if your system is suddenly restarted and *both* drives die before it boots. Then, any data that was not committed to the other disks, but still in SLOG will be lost. ZFS integrity should not be impacted and any older data should still be intact. Of course, bugs happen: the above is what should happen in absence of bugs. Daniel > > On Wed, Jul 9, 2014 at 2:27 AM, Daniel Kalchev <daniel@digsys.bg > <mailto:daniel@digsys.bg>> wrote: > > > On 08.07.14 04:06, javocado wrote: > > I am hoping to dumbly plug two SSDs onto motherboard SATA > ports and just > hand them over, raw, to ZFS. > > > Others already commented how you should setup mirrored SLOG. In > addition to that, because of the nature of SSDs and SLOG, I would > recommend the following: > > The SLOG size does not need to be large, it should only cover > several seconds of your synchronous write throughput -- usually > few GB are plenty. Today's SSDs are much large than needed for > SLOG. But, today's SSDs also suffer severe performance > degradation, especially for writing when you fill them up with > data and they need to do garbage collection. Also, most SSDs have > "good performance" only when using an 8GB span, not the whole > drive. All of this only makes sense if the drive has TRIM. FreeBSD > already supports TRIM for ZFS SLOG. Therefore ensure you do TRIM > of the entire drive, then partition it with GPT to only use (say) > 8GB for the SLOG. Leave the rest unallocated -- you will never > write there but the drive's controller will use those blocks as > spares for TRIM and this will both improve performance and make > the drive last much longer. Then add both slices as a mirrored log > device to your ZFS pool. > > Daniel > > _______________________________________________ > freebsd-fs@freebsd.org <mailto:freebsd-fs@freebsd.org> mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-fs > To unsubscribe, send any mail to > "freebsd-fs-unsubscribe@freebsd.org > <mailto:freebsd-fs-unsubscribe@freebsd.org>" > >
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