Patch for inconsistent recording of block device statistics

Tue, 22 Mar 2005 13:56:06 -0800

The read/write statistics for both sectors and merges are calculated at the time requests first enter the request queue but the remainder of the statistics, such as the number of read/writes are calculated at the time the I/O completes. As a result, one cannot accurately determine the data rates read or written at the actual time the I/O is performed. This behavior is masked with smaller queue sizes but is very real and was very noticeable with earlier 2.6 kenels using the cfq scheduler which had a default queue size of 8192 where the time difference between these sets of counters could exceed 10 seconds for large file writes and small monitoring intervals such as 1 second. In that environment, one would see extremely high bursts of I/O, sometimes exceeding 500 or even 1000 MB/sec for the first second or two and then drop to 0 for a long time while the 'number of operations' counters accurately reflect what is really happening.

The attached patch fixes this problem by simply accumulating the read/write sector/merge data in temporary variables stored in the request queue entry, and when the I/O completes copies those values to the disk statistics block.

-mark

diff -uprN -X dontdiff ../linux-2.6.11.4/drivers/block/ll_rw_blk.c ../linux-2.6.11.4-mjs/drivers/block/ll_rw_blk.c
--- ../linux-2.6.11.4/drivers/block/ll_rw_blk.c 2005-03-15 19:09:00.000000000 -0500
+++ ../linux-2.6.11.4-mjs/drivers/block/ll_rw_blk.c 2005-03-22 15:43:07.000000000 -0500
@@ -2107,13 +2107,13 @@ void drive_stat_acct(struct request *rq,
return;

    if (rw == READ) {
-        __disk_stat_add(rq->rq_disk, read_sectors, nr_sectors);
+                rq->read_sectors_accum += nr_sectors;
        if (!new_io)
-            __disk_stat_inc(rq->rq_disk, read_merges);
+                        rq->read_merges_accum += 1;
    } else if (rw == WRITE) {
-        __disk_stat_add(rq->rq_disk, write_sectors, nr_sectors);
+                rq->write_sectors_accum += nr_sectors;
        if (!new_io)
-            __disk_stat_inc(rq->rq_disk, write_merges);
+                        rq->write_merges_accum += 1;
    }
    if (new_io) {
        disk_round_stats(rq->rq_disk);
@@ -2487,6 +2487,11 @@ get_rq:
    req->rq_disk = bio->bi_bdev->bd_disk;
    req->start_time = jiffies;

+ req->write_sectors_accum=0;
+ req->write_merges_accum=0;
+ req->read_sectors_accum=0;
+ req->read_merges_accum=0;
+
add_request(q, req);
out:
if (freereq)
@@ -2989,10 +2994,14 @@ void end_that_request_last(struct reques
case WRITE:
__disk_stat_inc(disk, writes);
__disk_stat_add(disk, write_ticks, duration);
+ __disk_stat_add(disk, write_sectors, req->write_sectors_accum);
+ __disk_stat_add(disk, write_merges, req->write_merges_accum);
break;
case READ:
__disk_stat_inc(disk, reads);
__disk_stat_add(disk, read_ticks, duration);
+ __disk_stat_add(disk, read_sectors, req->read_sectors_accum);
+ __disk_stat_add(disk, read_merges, req->read_merges_accum);
break;
}
disk_round_stats(disk);
diff -uprN -X dontdiff ../linux-2.6.11.4/include/linux/blkdev.h ../linux-2.6.11.4-mjs/include/linux/blkdev.h
--- ../linux-2.6.11.4/include/linux/blkdev.h 2005-03-15 19:09:02.000000000 -0500
+++ ../linux-2.6.11.4-mjs/include/linux/blkdev.h 2005-03-22 15:42:47.000000000 -0500
@@ -176,6 +176,12 @@ struct request {
* For Power Management requests
*/
struct request_pm_state *pm;
+
+ /*
+ * accumulate intermediate stats
+ */
+ unsigned long read_sectors_accum, write_sectors_accum;
+ unsigned long read_merges_accum, write_merges_accum;
};

/*

-
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