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I would be thankfull if someone who understands how LVM works, could tell me a rough estimate, how much slower using LVM (with a Software RAID1) will be.

(What I do not want to know how much slower LVM will be if the LVM Volume is currently in snapshot mode doing Copy on Write). I only need some rough estmiate how much LVM will slow down reads and writes in a normal operation scenario.

Any links are also very much appreciated I was not able to find any good performance benachmarks about this question.

Thanks very much!! Jens

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up vote 7 down vote accepted

LVM is fairly lightweight for just normal volumes (without snapshots, for example). It's really just a table lookup in a fairly small table that block X is actually block Y on device Z. I've never done any benchmarking, but I've never noticed any performance differences between LVM and just using the raw device. It's some small extra CPU overhead on the disc I/O, so I really wouldn't expect much difference.

My gut reaction is that the reason there are no benchmarks is that there just isn't that much overhead in LVM.

The convenience of LVM, and being able to slice and dice and add more drives, IMHO, far outweighs what little (if any) performance difference there may be.

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hello sean, thanks for this helpfull answer! now I understand how lvm works and it seems reasonalbe that this table is held in memory and will incure only a very small overhead for the "in memory lookup". thanks for describing it that clearly. thanks! – jens Dec 7 '10 at 4:46
    
No problem, glad to help. Welcome to Server Fault. – Sean Reifschneider Dec 7 '10 at 12:58
    
I dunno, my experience with lvm isn't that impressive. I ran a little test, I put a 70 gig image on a raw ext4 volume and the same 70 gig image on an lvm 2 disk mirror formatted to ext4. Same machine same exact disks. I then dd'd the image to /dev/null, it took 6 minutes 9 seconds to read from ext4 and 16 minutes 1 second to read from the mirror. That's READing. – Stu Dec 7 '15 at 22:02
    
Did I mention reading, from a mirror? Should have gone faster than the plain ext4 disk. – Stu Dec 7 '15 at 22:02

with snapshot active lvm performs ... badly.

take a look here to see in-depth benchmark

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There is an excellent (be it old) whitepaper, written by a SUSE guy, about LVM and it's overhead here. It shows some (simple) benchmarks and explains the tech behind LVM. Good read.

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link is dead. :-( – Stu Dec 7 '15 at 21:59
    
This is the one: blyx.com/public/docs/Lvm.pdf – wzzrd Dec 9 '15 at 20:20
    
cool thanks. I'll check it out. – Stu Dec 9 '15 at 21:32

I am installing a 48T Dell MD-1200 and I was curious about this question. The MD1200 is connected to a hardware RAID card set up as RAID-6, so it looks to Linux like just a (big) drive. I tested an XFS filesystem on an LVM physical volume vs. an XFS filesystem on a straight disk partition. I used a Dell R630 machine with two E5-2699 CPUs in it. The system was set for Performance; whatever energy saving features I could find in the BIOS were turned off.

I installed CentOS 6.7 on it. Kernel is 2.6.32-573.el6.x86_64 (sorry for the oldie kernel but that's what I need for production). LVM is version 2.02.118.

I let CentOS create an XFS partition during the build. It is 1T in size. Then I created another 1T partition on the disk and created a logical volume:

vgcreate vol_grp1 /dev/sdb1
lvcreate -l 100%FREE -n lv_vol1 vol_grp1
mkfs.xfs /dev/vol_grp1/lv_vol1

My XFS-only filesystem was called /data_xfs. The LVM-backed XFS filesystem was called /data_lvm. I tested using bonnie++ v 1.03e.

The commands were: bonnie++ -u 0:0 -d /FILESYSTEM -s 400G -n 0 -m xfsspeedtest -f -b where FILESYSTEM was either /data_xfs or /data_lvm . Results are summarized as follows:

Test                        XFS on Partition        XFS on LVM
Sequential Output, Block    1467995 K/S, 94% CPU    1459880 K/s, 95% CPU
Sequential Output, Rewrite   457527 K/S, 33% CPU     443076 K/S, 33% CPU

Sequential Input, Block      899382 K/s, 35% CPU     922884 K/S, 32% CPU

Random Seeks                 415.0 /sec.              411.9 /sec.

Results seemed comparable in my view. In the Sequential Input test, LVM actually seemed to perform a little better.

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