ITAGAKI Takahiro wrote:
Greg Smith <[EMAIL PROTECTED]> wrote:
On Mon, 11 Jun 2007, ITAGAKI Takahiro wrote:
If the kernel can treat sequential writes better than random writes, is it worth sorting dirty buffers in block order per file at the start of checkpoints?

I wrote and tested the attached sorted-writes patch base on Heikki's
ldc-justwrites-1.patch. There was obvious performance win on OLTP workload.

  tests                    | pgbench | DBT-2 response time (avg/90%/max)
 LDC only                  | 181 tps | 1.12 / 4.38 / 12.13 s
 + BM_CHECKPOINT_NEEDED(*) | 187 tps | 0.83 / 2.68 /  9.26 s
 + Sorted writes           | 224 tps | 0.36 / 0.80 /  8.11 s

(*) Don't write buffers that were dirtied after starting the checkpoint.

machine : 2GB-ram, SCSI*4 RAID-5
pgbench : -s400 -t40000 -c10  (about 5GB of database)
DBT-2   : 60WH (about 6GB of database)

Wow, I didn't expect that much gain from the sorted writes. How was LDC configured?

3) The OS disk elevator should be dealing with this issue, particularly because it may really know the actual disk ordering.

Yeah, but we don't give the OS that much chance to coalesce writes when we spread them out.

Here's the subtle thing: by writing in the same order the LRU scan occurs in, you are writing dirty buffers in the optimal fashion to eliminate client backend writes during BuferAlloc. This makes the checkpoint a really effective LRU clearing mechanism. Writing in block order will change that.

The issue will probably go away after we have LDC, because it writes LRU
buffers during checkpoints.

I think so too.

  Heikki Linnakangas

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