Date: Fri, 15 Feb 2013 12:01:23 +0100 From: Daniel Kalchev <daniel@digsys.bg> To: grarpamp <grarpamp@gmail.com> Cc: freebsd-fs@freebsd.org Subject: Re: Crazy ZFS ZIL options: md(4) umass(4) Message-ID: <F053D5FF-0833-4091-9C5D-6A4E10A81CD8@digsys.bg> In-Reply-To: <CAD2Ti28ZKhrf3Yo06ooL9NXChtSEgvE-Rv_FngY1nfnVwUZ3YQ@mail.gmail.com> References: <CAD2Ti2_9i3rj5763UCjzxRw_7%2BqDky1MNRJzdvOZnmdYpLUfYQ@mail.gmail.com> <CA%2BtpaK3wA4kNVzLfE9EDaR1SGLN7_t-N-9Sw%2BvcsdeUKX4EFoA@mail.gmail.com> <CAD2Ti28ZKhrf3Yo06ooL9NXChtSEgvE-Rv_FngY1nfnVwUZ3YQ@mail.gmail.com>
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Some further clarification. ZIL is only read on crash. If you shut down properly etc, the ZIL will = be never read. For most of it's lifetime, the ZIL is write-only safety = net. An insurance, if you wish. Because it is write-only, cheap USB FLASH devices are the poorest = candidates. USB FLASH devices, even the most "high performance" are = mediocre at best. Because SLOG (separate ZIL) is written sequentially, = even the slowest spinning disk has higher throughput. You want = "write-optimised" device for the SLOG, which no USB FLASH device is, = ever. By the way, you may have better luck with professional photography = write-optimised CF cards, that are capable of recording large amounts of = huge raw images in a burst -- but these typically are more expensive = than an write-optimised SSD and you don't really need much capacity. But = again: forget about USB if you don't enjoy problems. :) By separating the ZIL, you avoid fragmentation, yes. This is because = in-pool ZIL uses variable length records, that are then removed, leaving = variable length holes. ZFS tries to work around this, but it's never = perfect. If you use separate mirrored disks for the ZIL, you will=20 - keep writing sequential (no seeks from interfering reads); - have a reliable recovery medium for the ZIL as long as at least one of = the drives survives the outage. If you want ultimate performance, the solution is battery backed RAM = drives for ZIL. Daniel
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