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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

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