General discussion of the Centos Linux distribution.

Smithies, Russell | 20 Aug 2012 00:16

LVM overhead? Does it cripple I/O?

For a high-performance system (64-cores, 512GB RAM, 5TB local disk, 110TB NFS-mounted storage) is there
any advantage of dropping lvm and mounting partitions directly?
We're not planning on changing partition sizes, but if we did we'd probably do a full rebuild.
Has anyone done performance testing to show that lvm isn't crippling I/O?

Thanx,

Russell

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Nux! | 20 Aug 2012 00:22

Re: LVM overhead? Does it cripple I/O?

On 19.08.2012 23:16, Smithies, Russell wrote:
> For a high-performance system (64-cores, 512GB RAM, 5TB local disk,
> 110TB NFS-mounted storage) is there any advantage of dropping lvm and
> mounting partitions directly?
> We're not planning on changing partition sizes, but if we did we'd
> probably do a full rebuild.
> Has anyone done performance testing to show that lvm isn't crippling 
> I/O?
>
> Thanx,
>
> Russell

I'm a long time user of LVM and I never noticed any problems with it, 
let alone "crippling IO".
In some benchmarks[1] it even speeds up things and I believe the 
Anaconda automatic partitioning defaults to LVM, too.

[1] - 
http://www.phoronix.com/scan.php?page=article&item=fedora_15_lvm&num=3

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Sent from the Delta quadrant using Borg technology!

Nux!
www.nux.ro

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John R Pierce | 20 Aug 2012 00:34

Re: LVM overhead? Does it cripple I/O?

On 08/19/12 3:16 PM, Smithies, Russell wrote:
> For a high-performance system (64-cores, 512GB RAM, 5TB local disk, 110TB NFS-mounted storage) is there
any advantage of dropping lvm and mounting partitions directly?
> We're not planning on changing partition sizes, but if we did we'd probably do a full rebuild.
> Has anyone done performance testing to show that lvm isn't crippling I/O?

running an extensive series of pgbench (postgresql benchmark similar to 
tps-c) with and without lvm and using ext4 vs xfs, on a very large 
raid10 built from 16 drives, the performance differences were pretty 
much non-existant, down in the noise, less than 1%.   this was on a 12 
core (24 thread) 3Ghz 48GB ram system, using a LSI logic megasas2 raid 
card with 512MB battery backed writeback cache.   the IO in these tests 
were 99% random write, and the tests were run with all 4 combinations of 
LVM or not and EX4 vs XFS and with different client connection counts 
(typically up around 100 client connections was where we got the peak 
transaction throughput

the entire raid was dedicated to the database tests,  the OS was running 
on a seperate raid1 which had pretty much zero IO during the benchmark 
sequences.

now, this isn't saying that you might not get a totally different result 
under different test conditions.

--

-- 
john r pierce                            N 37, W 122
santa cruz ca                         mid-left coast