I have been experimenting with empirical tests of file system and device
level writes to determine the actual constraints in order to speed up the WAL
Using a raw file partition and a time-based technique for determining the
optimal write position, I am able to get 8K writes physically written to disk
synchronously in the range of 500 to 650 writes per second using FreeBSD raw
device partitions on IDE disks (with write cache disabled). I will be
testing it soon under linux with 10,00RPM SCSI which should be even better.
It is my belief that the mechanism used to achieve these speeds could be
incorporated into the existing WAL logging code as an abstraction that looks
to the WAL code just like the file level access currently used. The current
speeds are limited by the speed of a single disk rotation. For a 7,200 RPM
disk this is 120/second, for a 10,000 RPM disk this is 166.66/second
The mechanism works by adjusting the seek offset of the write by using
gettimeofday to determine approximately where the disk head is in its
rotation. The mechanism does not use any AIO calls.
Assuming the following:
1) Disk rotation time is 8.333ms or 8333us (7200 RPM).
2) A write at offset 1,500K completes at system time 103s 000ms 000us
3) A new write is requested at system time 103s 004ms 166us
4) A 390K per rotation alignment of the data on the disk.
5) A write must be sent at least 20K ahead of the current head position to
ensure that it is written in less than one rotation.
It can be determined from the above that a write for an offset of something
slightly more than 195K past the last write, or offset 1,695K will be ahead
of the current location of the head and will therefore complete in less than
a single rotation's time.
The disk specific metrics (rotation speed, bytes per rotation, base write
time, etc.) can be derived empirically through a tester program that would
take a few minutes to run and which could be run at log setup time.
The obvious problem with the above mechanism is that the WAL log needs to be
able to read from the log file in transaction order during recovery. This
could be provided for using an abstraction that prepends the logical order
for each block written to the disk and makes sure that the log blocks contain
either a valid logical order number or some other marker indicating that the
block is not being used.
A bitmap of blocks that have already been used would be kept in memory for
quickly determining the next set of possible unused blocks but this bitmap
would not need to be written to disk except during normal shutdown since in
the even of a failure the bitmaps would be reconstructed by reading all the
blocks from the disk.
Checkpointing and something akin to log rotation could be handled using this
mechanism as well.
So, MY REAL QUESTION is whether or not this is the sort of speed improvement
that warrants the work of writing the required abstraction layer and making
this very robust. The WAL code should remain essentially unchanged, with
perhaps new calls for the five or six routines used to access the log files,
and handle the equivalent of log rotation for raw device access. These new
calls would either use the current file based implementation or the new
logging mechanism depending on the configuration.
I anticipate that the extra work required for a PostgreSQL administrator to
use the proposed logging mechanism would be to:
1) Create a raw device partition of the appropriate size
2) Run the metrics tester for that device partition
3) Set the appropriate configuration parameters to indicate raw WAL logging
I anticipate that the additional space requirements for this system would be
on the order of 10% to 15% beyond the current file-based implementation's
So, is this worth doing? Would a robust implementation likely be accepted for
7.4 assuming it can demonstrate speed improvements in the range of 500tps?
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