Re: Linux 2.6.29

Mark Lord
2009-03-30T16:30:30Z

Linus Torvalds wrote:
>
> On Mon, 30 Mar 2009, Ric Wheeler wrote:
>> A modern S-ATA drive has up to 32MB of write cache. If you lose power or
>> suffer a sudden reboot (that can reset the bus at least), I am pretty sure
>> that your above assumption is simply not true.
>
> At least traditionally, it's worth to note that 32MB of on-disk cache is
> not the same as 32MB of kernel write cache.
>
> The drive caches tend to be more like track caches - you tend to have a
> few large cache entries (segments), not something like a sector cache. And
> I seriously doubt the disk will let you fill them up with writes: it
> likely has things like the sector remapping tables in those caches too.
..

I spent an entire day recently, trying to see if I could significantly fill
up the 32MB cache on a 750GB Hitach SATA drive here.

With deliberate/random write patterns, big and small, near and far,
I could not fill the drive with anything approaching a full second
of latent write-cache flush time.

Not even close. Which is a pity, because I really wanted to do some testing
related to a deep write cache. But it just wouldn't happen.

I tried this again on a 16MB cache of a Seagate drive, no difference.

Bummer. :)