I don't have a deep knowledge about filesystems and I've few experiences
raid configuration, so I need someone who can answer some questions about
write back cache and barriers.
I have two sata hd (sda, sdb) configured in bios-raid 0 and relative device
created by dmraid on boot (nvidia_ihfaaicb).
(1) what should I do to know if the write back cache is enabled or not ?
lots of this is in the faq:
(2) if the write cache is enabled on both sda and sdb this mean that is
also on the raid device (/dev/mapper/nvidia_ihfaaicb) ?
I'd guess that if there's a write cache anywhere under your device, then in
effect the raid device would be considered write-cached.
(3) how should I do to enable/disable the write cache on the raid device ?
If I try mounting a xfs filesystem I get a message like "barriers are not
supported by this device" but if I mount a ext3 or a reiserfs filesystem
respectively with options barrier=1 and barrier=flush they don't complain.
If I mount the reiserfs I explicity get a message like "using barriers".
So who tells the truth ??
I don't see ext3 or reiser actually checking whether the underlying device
supports barriers. xfs does, in xfs_mountfs_check_barriers().
(4) is there a way to know if the raid device supports barrier or not ?
I'm not sure how barriers interact with multiple devices; perhaps someone else
(5) is there a (not dangerous) test I can do to figure out if barriers are
really enabled and used with ext3 and reiserfs filesystems ?
Thanks for reading the message