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RE: [linux-lvm] LVM performance




-- Steve Wray <steve wray the net nz>


From: linux-lvm-admin sistina com [mailto:linux-lvm-admin sistina com]On
Behalf Of Bradley M Alexander
On Fri, Nov 23, 2001 at 05:32:07PM +1300, Steve Wray wrote:
>
> thats dead right; ide can't simultaneously write to the master and
> slave on the same controller. Also, the pair of drives uses the
> controller circuitry
> of the master, so it always pays to make the master the most modern
> one.

Since both controllers have a drive master and a cd slave, this shouldn't
matter, per se.

yeah but I thought I'd mention it, for completeness :)

Dependong on the bios, for example, there still could be contention on the SB chipset for access to the controllers. Point is that there can be multiple bottlenecks.

> > Can anyone give me any ideas as to why the machine gets
beaten about so
> > much during IO operations and more importantly how can I minimize the
> > impact.
>
> Dunno, if it was because the fs is striped across those drives then
> splitting them across controllers would have made it go away.

Nothing should be striped across the drives. I have two PVs and made two
VGs, one on each drive. The way it works out, the data I am
mastering is on

yup you have to tell it to stripe, as I recall. Never used that feature tho.

I'm using LVM with several striped volumes. Hardware is Tekram 160MB scsi + pair of 80MB drives. The striping works rather nicely: sustained I/O is high for large copies and latency is low for reads. The file system uses a 4KB page w/ the inodes reduced to a reasonable value (e.g., mkfs.ext2 -b4096 -i10240).

If you are using the default of 1block / page then it can
hammer a system due to 2ndary access and higher write rate.

> I've seen no performance problems at all and really thrashed an
> LVM-root machine for test purposes while working on a movie. It took
> it well, performance-wise. (reliability is another issue; never go
> LVM-root... but thats just my 2 cents, YMMV).

Yeah, I wasn't brave enough to go all out...And with the problems I was
having, I'm glad I made that decision. :)

I think the worst part is the lack of backward compatibility at some stages of LVM. With root on LVM it can be a bit dangerous to upgrade! Or at least it was...

<broken-record> This can also be a real pain to fix if something goes wrong. If the lvm is --prefix-ed to, say, /lvm and the root volume is on a paritition then you can fix most things w/o having to play with rescue file systems. </broken-record>

--
Steven Lembark                               2930 W. Palmer
Workhorse Computing                       Chicago, IL 60647
                                           +1 800 762 1582



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