Re: -F option

On Mon, Dec 11, 2000 at 06:45:22PM -0500, newsreader(at)mediaone(dot)net wrote:
> manual page states that
>
> -F Disable an automatic fsync() call after each transaction. This option
> improves performance, but an operating system crash while a transaction is
> in progress may cause the loss of the most recently entered data. Without
> the fsync() call the data is buffered by the operating system, and written
> to disk sometime later.
>
> What I would like to know is what 'sometime later' means.
> Is it one hour? 30 seconds? 30 minutes? 24 hours? I really don't
> mind losing the last 3 minutes or so of data. If we are talking about 10
> hours or so then I will not use that switch. I'm not looking
> for an answer with milisecond accuracy; just an upper bound +- 5 minutes
> will be ok.

Read "man fsync()".

The answer is, "When your operating system gets around to it." The answer
depends on a great many factors: hard drive speed, drive bus speed (SCSI
or IDE), RAID overhead, if any, file system overhead, processor
loading. Whether the proper sacrifices to Murphy were made. Just to list
the ones that come easily to mind.

Having written disk drivers, I can tell you that you should be OK with a 5
minute latency requirement. In fact, on a reasonably loaded system, I
would hope all the data would be written within a minute. But I shan't be
greatly concerned unless latency is routinely greater than a minute.

I am assuming these are disk writes. With network writes (say, via NFS)
all bets are off.

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