Piergiorgio Sartor wrote:
I've a bunch of RAID-6 volumes, made of USB disks, i.e. using
PATA<->USB bridges.
Don't your bridges ever drop out or break?
What is the brand/model?
Years ago I broke alot of those, just by using the disks intensively.
They probably kinda overheated then failed. They couldn't last 2 days on
intense disk activity... They were chinese stuff bought on ebay though.
This volumes are aggregated using LVM and, on top of that, there
is a LUKS container.
The raw read perfomance on the RAID-6 is, in the best case,
around about 48MB/s, which is pretty good for USB, I guess it
will be difficult to get more.
48MB/sec can be good for 1 disk, but it's bad for many disks attached
separately to USB ports...
The raw read perfomance of the LVM volume is i~38MB/s.
The raw read performance of the LUKS is ~28MB/s (actually
maybe a bit less).
Might your LVM or partition within it be not aligned, or you didn't set
readahead?
http://www.beowulf.org/archive/2007-May/018359.html
http://www.mail-archive.com/linux-raid@xxxxxxxxxxxxxxx/msg10804.html
People using LVM on arrays giving hundreds of MB/sec see slowdowns of
the order of percent
http://article.gmane.org/gmane.linux.raid/18302
LVM also does not merge requests AFAIR. (visible with mdstat -x 1)
BTW, what's that? I mean "mdstat -x 1"...
I'm sorry I meant " iostat -x 1"
But me also I have never seen mismatches and the only raid-1's I
have are above LVM. (except /boot but that's almost never modified)
Well, that's good, you confirmed my experience.
I've also RAID-10 on LVM and never got mismatches, while the
plain RAID-10 got sometimes.
This fact needs further investigation methinks...
We could ask to the LVM people if LVM really copies the buffer.
Regards
A.
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