>WARNING: modern TOtL flash RAMs are only good for ~1.2M writes per
>memory cell. and that's the =good= ones.
>Using flash RAM for write heavy applications like OLTP, or for WAL,
>etc can be very dangerous
Well, that's why I suggested that the WAL would stream to a hard disk
array, where the large IO sequential write speed will be helpful.
Whether OLTP is a problem will presumably depend on the freqency of updates
and vacuum to each physical cluster of rows in a disk block.
Most rows in a trading application will have quite a long lifetime, and be
updated relatively few times (even where we writing fixings info into
trades).
>Flash write speeds also stink; being ~1/2 flash's already low read speed.
Sure - but it may still be an effective tradoff where the limiting factor
would otherwise be seek time.
>Much better to use flash RAM for read heavy applications.
Why? I can get a 'PC' server with 128GB of RAM quite easily now,
and that will mean I can cache most of not all hot data for any trading
app I've worked on. Settled trades that matured in prior periods can
be moved to tables on real disks - they are hardly ever accessed
anyway.
In the long run, we are going to have to seriously rethink pg's use
of WAL as the way we implement MVCC as it becomes more and more of a
performance bottleneck.
We have WAL because Stonebreaker made an assumption about the future
dominance of optical media that has turned out to be false.
...and it's been one of pg's big issues every since.
>> 2GB CF isn't so
>>pricey any more.
>Heck =16= GB Flash only costs ~$300 US and 128GB SSDs based on flash
>RAM are due out this year.
Quite. Suppose I have a RAID with double redundancy, then I get enough
capacity
for quite a lot of raw data, and can swap a card out every weekend and let
the
RAID rebuild it in rotation to keep them within conservative wear limits.
So long as the wear levelling works moderately well (and without needing FAT
on the disk or whatever) then I should be fine.
I think. Maybe.
James
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