[gentoo-server] It's amazing what switching the IO scheduler can do.

[David Miller]

I just had a break through with my main file server here in it's
ability to handle a high load with lots of read/writes going on
simultaneously so I figured I would pass this on in hopes that it will
help someone with a similar problem.

The setup:
  Dual Xeon 2.8ghz, 2gb ram, LSI 320 pci-x dual channel scsi
controller with about 18tb of SATA over SCSI attached storage all in
RAID 5.  All storage is managed using LVM2.

The workload:
  This server does alot more reads than writes and is almost always
serving files over the network (copper Gigabit) at rates of 100 to 110
meg/s.  This is done using samba as the client PC's are windows based.

The problem:
  When server was serving files at high rates any writes would drive
up IO waits and server load to the point that samba would become
unhappy.  In some cases the entire SCSI io system in the kernel would
become unhappy causing scsi bus resets, file systems to unmount etc. 
Luckily this never resulted in any data loss or corruption but it has
stopped some of the processing that we were doing on the files.

My solution:
  The solution seems to be CFQ IO scheduling.  I had previously used
Deadline and the Anticipatory scheduler but neither of these solved
the problem.  CFQ seems to keep everything happy even under a mixed
I/O load of multiple writes (35mb/s or so) and multiple I/O reads
(70mb/s or so).  The server load is still high in the 12 to 14 range
but the server and it's services seem to be much more stable.

So what type of IO Scheduler have you found to handle your typical
load the best?  Are there any other IO Schedulers that haven't made it
to the mainstream source trees yet that are worth a look?  How long
until we have a IO scheduler that can adjust it self to deal with
various loads dynamically...I'm sure this is an ongoing area of
research.

-David

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