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[linux-lvm] Aligning PVs on SSDs?


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?


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
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roy karlsbakk net
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