Re: Linux 2.6.29
From: Mark Lord
Date: Mon Mar 30 2009 - 12:30:31 EST
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. :)
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