[patch] jbd: tag journal writes as metadata I/O

From: Jeff Moyer
Date: Tue Apr 06 2010 - 12:53:10 EST


In running iozone for writes to small files, we noticed a pretty big
discrepency between the performance of the deadline and cfq I/O
schedulers. Investigation showed that I/O was being issued from 2
different contexts: the iozone process itself, and the jbd/sdh-8 thread
(as expected). Because of the way cfq performs slice idling, the delays
introduced between the metadata and data I/Os were significant. For
example, cfq would see about 7MB/s versus deadline's 35 for the same
workload. I also tested fs_mark with writing and fsyncing 1000 64k
files, and a similar 5x performance difference was observed. Eric
Sandeen suggested that I flag the journal writes as metadata, and once I
did that, the performance difference went away completely (cfq has
special logic to prioritize metadata I/O).

This is similar to the jbd2 patch I posted earlier that Ted has accepted
into his patch queue. Comments, as always, are appreciated.

Signed-off-by: Jeff Moyer <jmoyer@xxxxxxxxxx>

diff --git a/fs/jbd/commit.c b/fs/jbd/commit.c
index 4bd8825..e9f1369 100644
--- a/fs/jbd/commit.c
+++ b/fs/jbd/commit.c
@@ -318,7 +318,7 @@ void journal_commit_transaction(journal_t *journal)
int first_tag = 0;
int tag_flag;
int i;
- int write_op = WRITE;
+ int write_op = WRITE_META;

* First job: lock down the current transaction and wait for
@@ -357,7 +357,7 @@ void journal_commit_transaction(journal_t *journal)
* instead we rely on sync_buffer() doing the unplug for us.
if (commit_transaction->t_synchronous_commit)
- write_op = WRITE_SYNC_PLUG;
+ write_op = WRITE_SYNC_PLUG | (1<<BIO_RW_META);
while (commit_transaction->t_updates) {
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