On 08/31/2009 11:50 AM, david@xxxxxxx wrote:
On Mon, 31 Aug 2009, Ric Wheeler wrote:
On 08/31/2009 09:16 AM, Christoph Hellwig wrote:
On Mon, Aug 31, 2009 at 09:15:27AM -0400, Ric Wheeler wrote:
While most common filesystem do have barrier support it is:
- not actually enabled for the two most common filesystems
- the support for write barriers an cache flushing tends to be buggy
all over our software stack,
Or just missing - I think that MD5/6 simply drop the requests at
present.
I wonder if it would be worth having MD probe for write cache enabled&
warn if barriers are not supported?
In my opinion even that is too weak. We know how to control the cache
settings on all common disks (that is scsi and ata), so we should always
disable the write cache unless we know that the whole stack (filesystem,
raid, volume managers) supports barriers. And even then we should make
sure the filesystems does actually use barriers everywhere that's needed
which failed at for years.
I was thinking about that as well. Having us disable the write cache
when we know it is not supported (like in the MD5 case) would
certainly be *much* safer for almost everyone.
We would need to have a way to override the write cache disabling for
people who either know that they have a non-volatile write cache
(unlikely as it would probably be to put MD5 on top of a hardware
RAID/external array, but some of the new SSD's claim to have
non-volatile write cache).
I've done this when the hardware raid only suppored raid 5 but I wanted
raid 6. I've also done it when I had enough disks to need more than one
hardware raid card to talk to them all, but wanted one logical drive for
the system.
It would also be very useful to have all of our top tier file systems
enable barriers by default, provide consistent barrier on/off mount
options and log a nice warning when not enabled....
most people are not willing to live with unbuffered write performance.
they care about their data, but they also care about performance, and
since performance is what they see on an ongong basis, they tend to care
more about performance.
given that we don't even have barriers enabled by default on ext3 due to
the performance hit, what makes you think that disabling buffers
entirely is going to be acceptable to people?
David Lang
We do (and have for a number of years) enable barriers by default for XFS and
reiserfs. In SLES, ext3 has default barriers as well.
Ric
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