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@karlsbakk.net
http://blogg.karlsbakk.net/
--
I all pedagogikk er det essensielt at pensum presenteres
intelligibelt. Det er et elementært imperativ for alle pedagoger å
unngå eksessiv anvendelse av idiomer med fremmed opprinnelse. I de
fleste tilfeller eksisterer adekvate og relevante synonymer på norsk.
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