On Fri, 16 Jan 2015, Dave Caplinger wrote:
On Jan 16, 2015, at 2:51 PM, David Lang <da...@lang.hm> wrote:
On Fri, 16 Jan 2015, Dave Caplinger wrote:
... filesystem buffer can help speed reading data previously written to
disk if your outage was short enough to not get "too far" behind, because the
data is still actually in RAM so you don't actually have to pay physical IOPS
to touch the disk to retrieve it.
the filesystem actions are the super expensive parts, even if things are cached
to ram. There are also fsyncs that take place to make the data safe, and they
force disk IOPS
I agree the write path is certainly expensive (and more so by frequent fsyncs),
but when you come back 'n' minutes later to read it (and it's still in the
filesystem buffer), I only meant that it's much quicker than having to actually
seek and read from disk again. So you're not paying the penalty twice in this
case.
the read path is also expensive, because as you read the messages from the
filesystem cache, you are also doing filesystem operations to make the messages
that you are reading as being processed (which takes several steps), it's not
just "read from disk", it'a more like "mark these messages as being worked on",
"read the messsages and process them", "mark these messages as processed", for
every message (with some savings for batching, but probably less than you would
think)
David Lang
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