On 29/10/2011 07:26, NeilBrown wrote:
On Sat, 29 Oct 2011 08:49:09 +0400 CoolCold<coolthecold@xxxxxxxxx> wrote:
[...]
As there are some pros and cons on both sides (at least theoretically) I have dumb question - let's say our array md1 consists on 3 drives - /dev/sd{a,b,c} - and when data read from md1 occurs, which block is cached in VFS (or may be other cache in system, it would be nice to know which part of system is doing caching) - the block from md1 itself or from certain drive? If it is drive-based block cache, it's gonna be potentially memory wasting to keep 3 similar data copies, so I assume md does data reads with something like O_DIRECT flag, but as I 1) don't know C 2) don't know kernel, I'm asking this on the list to make this clean for myself.The kernel caches pages of files, not pages of devices. It doesn't matter where the page of data came from - it is the page of a file that is cached.
I suppose if the user was silly enough to mount the same filesystem from both md1 and sd{a,b,c} simultaneously then there could be duplication of caching, but as I say I think that'd be a silly configuration :-)
Cheers, John. -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
