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Re: [dm-devel] [PATCH v6 0/4] ext4: Coordinate data-only flush requests sent by fsync



On 11/29/2010 05:05 PM, Darrick J. Wong wrote:
On certain types of hardware, issuing a write cache flush takes a considerable
amount of time.  Typically, these are simple storage systems with write cache
enabled and no battery to save that cache after a power failure.  When we
encounter a system with many I/O threads that write data and then call fsync
after more transactions accumulate, ext4_sync_file performs a data-only flush,
the performance of which is suboptimal because each of those threads issues its
own flush command to the drive instead of trying to coordinate the flush,
thereby wasting execution time.

Instead of each fsync call initiating its own flush, there's now a flag to
indicate if (0) no flushes are ongoing, (1) we're delaying a short time to
collect other fsync threads, or (2) we're actually in-progress on a flush.

So, if someone calls ext4_sync_file and no flushes are in progress, the flag
shifts from 0->1 and the thread delays for a short time to see if there are any
other threads that are close behind in ext4_sync_file.  After that wait, the
state transitions to 2 and the flush is issued.  Once that's done, the state
goes back to 0 and a completion is signalled.

Those close-behind threads see the flag is already 1, and go to sleep until the
completion is signalled.  Instead of issuing a flush themselves, they simply
wait for that first thread to do it for them.  If they see that the flag is 2,
they wait for the current flush to finish, and start over.

However, there are a couple of exceptions to this rule.  First, there exist
high-end storage arrays with battery-backed write caches for which flush
commands take very little time (<  2ms); on these systems, performing the
coordination actually lowers performance.  Given the earlier patch to the block
layer to report low-level device flush times, we can detect this situation and
have all threads issue flushes without coordinating, as we did before.  The
second case is when there's a single thread issuing flushes, in which case it
can skip the coordination.

This author of this patch is aware that jbd2 has a similar flush coordination
scheme for journal commits.  An earlier version of this patch simply created a
new empty journal transaction and committed it, but that approach was shown to
increase the amount of write traffic heading towards the disk, which in turn
lowered performance considerably, especially in the case where directio was in
use.  Therefore, this patch adds the coordination code directly to ext4.

Hi Darrick,

Just curious why we would need to have batching in both places? Doesn't your patch set make the jbd2 transaction batching redundant?

I noticed that the patches have a default delay and a mount option to override that default. The jbd2 code today tries to measure the average time needed in a transaction and automatically tune itself. Can't we do something similar with your patch set? (I hate to see yet another mount option added!)

Regards,

Ric



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