Re: raid(1) and block caching

[NeilBrown <neilb@suse.de>]

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On Sat, 29 Oct 2011 08:49:09 +0400 CoolCold <coolthecold@gmail.com> wrote:

> Hello!
> 
> There is holywar once again on nginx maillist about standalone drives
> vs raid1 arrays for serving static files. By standalone drives it is
> assumed that file "Filename1" exist on /mnt/disk1, /mnt/disk2,
> /mnt/diskN where /mnt/diskX is mountpoint for drives /dev/sdY.

If you want fast reads, then use RAID0 if you don't care about losing your
data, and RAID10 in 'far' mode if you want RAID protection.

> 
> 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.

NeilBrown

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