[Xen-API] [PATCH 5 of 5] CA-42914, SCTX-434: Speed up writes of the database to the redo-log

[Jonathan Davies <jonathan.davies@xxxxxxxxxx>]

# HG changeset patch
# User Jonathan Davies <jonathan.davies@xxxxxxxxxx>
# Date 1288100552 -3600
# Node ID 5cf0e1279a28ea8f2c4282594b1df39dd47cd6fa
# Parent  7e16add444a9b9387aa908f71570baee8c755981
CA-42914, SCTX-434: Speed up writes of the database to the redo-log

Anecdotal evidence suggests that the default 1 KiB block size provided by 
Unixext.read_in_chunks causes write throughput to be very slow on some storage 
substrates.

Some (not massively scientific) timings of dd from /dev/zero to a 
block-attached VDI on various SR types agree with this observation. The data 
below shows that 4 KiB is the minimum block size that should be considered for 
use: throughput it universally higher above this threshold and often 
substantially lower below it. We'll default to 16 KiB to be on the safe side.

lvmoiscsi

bs      count   times                   throughputs (MB/s)
1024    65536   0m8.461s, 0m7.888s      7.9, 8.5
2048    32768   0m7.986s, 0m8.438s      8.4, 8.0
4096    16384   0m1.529s, 0m1.999s      44.0, 33.6
16384   4096    0m1.289s, 0m1.588s      52.2, 42.3
65536   1024    0m1.550s, 0m1.309s      43.3, 51.3

nfs

bs      count   times                   throughputs (MB/s)
1024    65536   0m11.865s, 0m12.232s    5.7, 5.5
2048    32768   0m12.612s, 0m12.758s    5.3, 5.3
4096    16384   2.75591, 3.10301        24.4, 21.6
16384   4096    2.96047, 3.42373        22.7, 19.6
65536   1024    3.23713, 3.24731        20.7, 20.7

lvm

bs      count   times                   throughputs (MB/s)
1024    65536   2.16546, 6.35194        31.0, 10.6
2048    32768   6.24928, 6.44344        10.7, 10.4
4096    16384   0.99237, 0.90683        67.6, 74.0
16384   4096    0.912118, 0.88074       73.6, 76.2
65536   1024    0.934838, 0.881285      71.8, 76.1

Signed-off-by: Jonathan Davies <jonathan.davies@xxxxxxxxxx>

diff -r 7e16add444a9 -r 5cf0e1279a28 ocaml/database/block_device_io.ml
--- a/ocaml/database/block_device_io.ml Tue Oct 26 14:42:31 2010 +0100
+++ b/ocaml/database/block_device_io.ml Tue Oct 26 14:42:32 2010 +0100
@@ -324,7 +324,7 @@
         (* Otherwise write it *)
         Unixext.time_limited_write dest_fd len chunk target_response_time;
         total_length := !total_length + len
-      ) data_client
+      ) ~block_size:16384 data_client
     )
     (fun () -> 
       (* Close the connection *)

Attachment: xen-api.hg-5.patch
Description: Text Data

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