[linux-lvm] Re: Aligning PVs on SSDs?
snitzer at redhat.com
Sat Jul 11 13:51:44 UTC 2009
You want to use: pvcreate --dataalignment 128k /dev/sdXX
This will guarantee that the start of the data area (pe_start) is
aligned on a 128k boundary.
On Sat, Jul 11 2009 at 6:23am -0400,
Roy Sigurd Karlsbakk <roy at karlsbakk.net> wrote:
> I came across an article on
> 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
>> 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
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