[linux-lvm] Volume alignment over RAID
Linda A. Walsh
lvm at tlinx.org
Fri May 21 06:48:31 UTC 2010
Luca Berra wrote:
> On Thu, May 20, 2010 at 02:24:19PM -0700, Linda A. Walsh wrote:
>> I'm a bit unclear as to where some units are applied in my RAID setup, but
>> was wondering how LVM interacted, could be, or should be setup so that
>> created volumes would be aligned properly on top of a RAID disk.
> note that if your rig uses fairly recent software data alignment should
> happen automagically.
So I'm told, but I like to verify, I'm paranoid :-)
>> I'm using a RAID 'chunk' size of 64k as suggested by the RAID documentation
>> and am using 6 disks to create a RAID6, giving 4 units of data/stripe. Does
> I suppose by raid you mean md, so i wonder what documentation you were
> looking at?
Well, doc in 2 different raid controllers LSI and rocket raid both
suggest 64K as a unit size (forget, their exact term).
> I think 64k might be small as a chunk size, depending on your array size
> you probably want a bigger size.
Really? What are the trade offs? Array size well 6 disks and 4 of data.
> Then, since with a six drive raid 6 stripe size is always a power of 2,
> answers are easy :)
I like easy...figure 4 should divide into most things.
>> this mean my logical volume needs to be aligned on a 64K boundary, or a 256k
>> boundary? I.e. does 64k usually specify chunk/unit, or chunk/stripe?
> align to stripe size
>> What do I need to do to make sure my logical volumes always line up on RAID
>> stripe boundaries?
> make the volume group with pe size multiple of stripe size
>> I've been using default logical volume parameters, which I think use an
>> allocation size measured in Megabytes, so does that imply I'm automatically
>> aligned (as 64k and 256k both divide into 1 Meg)
It's 4.0M, so not a problem...but this next one is:
>> Or is some offset involved?
> pvs -o pv_name,pe_start
192.00K is listed as the start of each! GRR...why would that
be a default...I suppose it works for someone, but it's NOT a power of 2!
So each start is messed up. Is there a way I can change the default on a per
volume basis? ...(yes: --datalignment; if I'd just read manpage
(off to read manpage...thanks for the help....this is most unpleasant,
given I' just copied 1.3T (6 hours worth) of data on to this thing already...
looks like I was a bit too eager, but was out of disk space on old partition.
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