Re: [dm-crypt] dm-crypt flush-to-disk freezes

[Milan Broz <mbroz@redhat.com>]

On 08/22/2010 11:42 PM, Christoph Anton Mitterer wrote:
> On Sun, 2010-08-22 at 21:52 +0200, Arno Wagner wrote:
>> What do I lose with barriers off?
> data security ;)
> 
> 
> In case of the filesystem barriers (not the IO barriers, which are
> different AFAIK) they're used to make sure, that the COMMITS in the
> journal are written after the journal is correctly flushed out.

Slight confusion here.

FS uses flush (called by fsync for example), it is currently implemented
using IO barrier.
After this operation, FS code can be sure that preceding barrier
reached disk.
(Device-mapper internally waits for all IO to finish, processing always
one barrier at a time and queuing following requests.)

If you disable it, data simply reach disk later and e.g. unexpected
power loss can cause quite serious data loss.
But you probably see better performance.

So disabling barriers helps in your case? Then probably some tuning
of fs can help also.

(There is ongoing discussion about reimplementing barriers in block
layer, maybe it will slightly help here too.)

Milan