[linux-lvm] Aligning PVs on SSDs?
Roy Sigurd Karlsbakk
roy at karlsbakk.net
Sat Jul 11 10:23:37 UTC 2009
Hi
I came across an article on http://thunk.org/tytso/blog/2009/02/20/aligning-filesystems-to-an-ssds-erase-block-size/
about filesystem alignment. This mentions some issues about aligning
PVs in 128k bounderies. See snipped-out text from the article below.
Are there flags like these or similar in helping admins using SSDs
more efficiently?
roy
> ...
> So I created a 1 gigabyte /boot partition as /dev/sdb1, and
> allocated the rest of the SSD for use by LVM as /dev/sdb2. And
> that’s where I ran into my next problem. LVM likes to allocate 192k
> for its header information, and 192k is not a multiple of 128k. So
> if you are creating file systems as logical volumes, and you want
> those volume to be properly aligned you have to tell LVM that it
> should reserve slightly more space for its meta-data, so that the
> physical extents that it allocates for its logical volumes are
> properly aligned. Unfortunately, the way this is done is slightly
> baroque:
>
> # pvcreate –metadatasize 250k /dev/sdb2
> Physical volume “/dev/sdb2″ successfully created
>
> Why 250k and not 256k? I can’t tell you — sometimes the LVM tools
> aren’t terribly intuitive. However, you can test to make sure that
> physical extents start at the proper offset by using:
>
> # pvs /dev/sdb2 -o+pe_start
> PV VG Fmt Attr PSize PFree 1st PE
> /dev/sdb2 lvm2 – 73.52G 73.52G 256.00K
>
> If you use a metadata size of 256k, the first PE will be at 320k
> instead of 256k. There really ought to be an –pe-align option to
> pvcreate, which would be far more user-friendly, but, we have to
> work with the tools that we have. Maybe in the next version of the
> LVM support tools….
> ...
--
Roy Sigurd Karlsbakk
(+47) 97542685
roy at karlsbakk.net
http://blogg.karlsbakk.net/
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