> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- > boun...@opensolaris.org] On Behalf Of Don > > Continuing on the best practices theme- how big should the ZIL slog > disk be? > > The ZFS evil tuning guide suggests enough space for 10 seconds of my > synchronous write load- even assuming I could cram 20 gigabits/sec into > the host (2 10 gigE NICs) That only comes out to 200 Gigabits which = > 25 Gigabytes. > > I'm currently planning to use 4 32GB SSD's arranged in 2 2 way mirrors > which should give me 64GB of log space. Is there any reason to believe > that this would be insufficient (especially considering I can't begin > to imagine being able to cram 5 Gb/s into the host- let alone 20). > > Are there any guidelines on how much ZIL performance should increase > with 2 SSD slogs (4 disks with mirrors) over a single SSD slog (2 disks > mirrored).
I think the size of the ZIL log is basically irrelevant ... For example, I remember reading somewhere that the system refuses to use more than 50% of the size of RAM, yet, you can hardly even think about buying an SSD smaller than 32G. If you've got a 64G ram system, you're probably not going to use only a single SSD, just due to the fact that you've probably got dozens of disks attached, and you'll probably use multiple log devices striped just for the sake of performance. Improbability assessment aside, suppose you use something like the DDRDrive X1 ... Which might be more like 4G instead of 32G ... Is it even physically possible to write 4G to any device in less than 10 seconds? Remember, to achieve worst case, highest demand on ZIL log device, these would all have to be <32kbyte writes (default configuration), because larger writes will go directly to primary storage, with only the intent landing on the ZIL. To try and quantify this a little closer, suppose all your writes are 31K (worst case for typical setup) ... meaning they're as large as possible while still going to the log device instead of primary storage. Suppose you get 2000 IOPS (which is roughly typical according to my benchmarks) then you're writing a little less than 64Mbytes/sec, and you won't even come close to reaching 1G within 10 seconds. As a cross-check, assume it's a PCIE 2.0 x1 bus. This is 500 Mbytes/sec theoretical maximum. So in 10 seconds, 5Gbyte theoretical maximum. How about if you're using SAS 6Gbit devices, using the unrealistic assumption that you can write 6Gbits. Well, that's 750 Mbytes unrealistically high, so 7.5G in 10 seconds. Which I know to be not just unrealistic, but ridiculously overestimated, by at least one order of magnitude. So, although I don't have any physical machine to test or verify this on, I have a very educated guess which says even the smallest nonvolatile device will be more than you can use for your ZIL log. Size doesn't matter. Just speed. (and reliability, price, etc) _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
