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Re: journal on an ssd



Tobias Oetiker wrote:
Hi Chris,

Yesterday Chris Worley wrote:

Note that I do have one to experiment with.
What's a good way to measure journal performance, and/or in what cases do
you need a faster journal (i.e. an EXT3 atop an MD device with big block
stripes)?

Chris

Well, the 'problem' we had to solve was the following:

setup:

- large HW raid6 array
- lvm on top
- many ext3 partitions

when there was a lot of write or meta data update activity on one
partition, performance on all other partitions went to 0.
(processes hanging for 10-20 seconds as soon as they accessed the
filesystem). I am sure that there is a bad-bad bug in the linux
kernel somewhere which is causing this, but all the upgrading and
patching did not help, the condition remained.
I assume that you have a hardware RAID card, not an external array? If you do have an array (IBM Shark, EMC box, etc) with battery backed internal cache, then you should get better than SSD speeds from one of its LUNs assuming your cache is large enough ;-)

ric

Until we moved the journals off to that external ssd.

Now I can copy partition A over to partition B and the server
remains nicely responsive. I am atributing that to the external
journal.

Obviously I would like to know how bad we are going to be had when
the ssd dies.

cheers
tobi




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