Re: Block-level CRC checks

On Thu, Oct 2, 2008 at 12:05 PM, Aidan Van Dyk <aidan(at)highrise(dot)ca> wrote:
> How does your current "write" strategy handle this situation. I mean,
> how do you currently guarnetee that between when you call write() and
> the kernel copies the buffer internally, no hint-bit are updated?

Working on the exact double-buffering technique now.

> #define write(fd, buf, count) buffer_crc_write(fd, buf, count)

I certainly wouldn't interpose the write() call itself; that's just
asking for trouble.

> whatever protection you have on the regular write is sufficient. The
> time of the protection will need to start before the "buffer" period
> instead of just the write, (and maybe not the write syscall anymore) but
> with CPU caches and speed, the buffer period should be <= the time of
> the write() syscall... Your fsync is your "on disk guarentee", not the
> write, and that won't change.

Agreed.

> But I thought you didn't really care about hint-bit updates, even in the
> current strategy... but I'm fully ignorant about the code, sorry...

The current implementation does not take it into account.

--
Jonah H. Harris, Senior DBA
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