Hi!
> > (actually write barriers should be enabled for all those AoE devices
> > --
> > especially with newer kernels.)
> >
>
>
> How?
>
The default behavior depends on the kernel version and the vendor (Redhat
is said to disable barrier support for local file systems on recent
kernels). Between 2.6.31 and 2.6.33 most/all devices gained propper barrier
support (which of course made disc access in most/all cases slower).
In case barrier support for the underlaying device is available, the mount
option "barrier" can be used to enable/disable barrier support. You can
for example disable barrier support with this command:
mount -o remount,barrier=0 /mount/point
For mounting file systems over a SAN protocol like AoE or iscsi I'd
strongly recommend using write barriers. Due to the higher latency of those
protocols ending up with a broken filesystem and lost data is way more
likely.
> > One data block of this RAID can only be written at once. So whenever
> > only
> > one bit within that block changes, the whole block has to written again
> >
>
>
> The alignment issues at every layer of the storage system have always
> been my
> biggest hassle in dealing with SANs.
>
Sigh. Yeah... it is not so easy to deal with that. I'm struggling myself
from time to time. ;-)
Probably time to write a complete tutorial on how to deal with alignment?!
-- any volunteers?? :-)
-- Adi
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