On Mon, Sep 21 2009 at 12:30pm -0400, Jon Hardcastle <jd_hardcastle@xxxxxxxxx> wrote: > --- On Mon, 21/9/09, Mike Snitzer <snitzer@xxxxxxxxx> wrote: > > > From: Mike Snitzer <snitzer@xxxxxxxxx> > > Subject: Re: LVM and Raid5 > > To: "Michal Soltys" <soltys@xxxxxxxx> > > Cc: "Linux Raid Study" <linuxraid.study@xxxxxxxxx>, linux-raid@xxxxxxxxxxxxxxx, linux-lvm@xxxxxxxxxx > > Date: Monday, 21 September, 2009, 3:33 PM > > On Thu, Sep 17, 2009 at 8:37 AM, > > Michal Soltys <soltys@xxxxxxxx> > > wrote: > > > Linux Raid Study wrote: > > >> > > >> Hello: > > >> > > >> Has someone experimented with LVM and Raid5 > > together (on say, 2.6.27)? > > >> Is there any performance drop if LVM/Raid5 are > > combined vs Raid5 alone? > > >> > > >> Thanks for your inputs! > > > > > > Few things to consider when setting up LVM on MD > > raid: > > > > > > - readahead set on lvm device > > > > > > It defaults to 256 on any LVM device, while MD will > > set it accordingly to > > > the amount of disks present in the raid. If you do > > tests on a filesystem, > > > you may see significant differences due to that. YMMV > > depending on the type > > > of used benchmark(s). > > > > > > - filesystem awareness of underlying raid > > > > > > For example, xfs created on top of raid, will > > generally get the parameters > > > right (stripe unit, stripe width), but if it's xfs on > > lvm on raid, then it > > > won't - you will have to provide them manually. > > > > > > - alignment between LVM chunks and MD chunks > > > > > > Make sure that extent area used for actual logical > > volumes start at the > > > boundary of stripe unit - you can adjust the LVM's > > metadata size during > > > pvcreate (by default it's 192KiB, so with non-default > > stripe unit it may > > > cause issues, although I vaguely recall posts that > > current LVM is MD aware > > > during initialization). Of course LVM must itself > > start at the boundary for > > > that to make any sense (and it doesn't have to be the > > case - for example if > > > you use partitionable MD). > > > > All of the above have been resolved in recent LVM2 > > userspace (2.02.51 > > being the most recent release with all these > > addressed). The last > > issue you mention (partitionable MD alignment offset) is > > also resolved > > when a recent LVM2 is coupled with Linux 2.6.31 (which > > provides IO > > Topology support). > > > > Mike > > -- > > When you say 'resolved' are we talking automatically? if so, when the > volumes are created... etc etc? Yes, automatically when the volumes are created. The relevant lvm.conf options (enabled by default) are: devices/md_chunk_alignment (useful for LVM on MD w/ Linux < 2.6.31) devices/data_alignment_detection devices/data_alignment_offset_detection readahead defaults to "auto" in lvm.conf: activation/readahead -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
