Re: 500 tpsQL + WAL log implementation

I kept this around from November because I wanted to think about it
further.

Basically, right now when we continually add to the last WAL page, we
have to wait for the platter to rotate to that block each time to do the
fsync.

the idea is to have multiple versions of the last WAL block, meaning you
write the first record of the last block, then when you want to write
another, your disk platter has moved, so you write the first and second
records in a new location.

If we could somehow know the platter location, or tell the disk drive to
write it in several locations, whichever is closest, I think we would
have a real win. However, I don't see any way of doing that.

Imagine what we could do with 8mb of battery-backed RAM! This is sort
of what we are using WAL/disk for, and it really isn't very good at it.

Is this a TODO item?

Find a way to reduce rotational delay when repeatedly writing
last WAL page

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Curtis Faith wrote:
> 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
> logging code.
>
> 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
> requirements.
>
> 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?
>
> - Curtis
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--
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