[systemd-devel] [PATCH] [RFC][PLEASE TEST] readahead: chunk on spinning media

[Kay Sievers]

On Sat, Mar 23, 2013 at 11:47 PM, Lennart Poettering
<lennart at poettering.net> wrote:
> On Sat, 23.03.13 23:42, Tom Gundersen (teg at jklm.no) wrote:
>
>>
>> On Sat, Mar 23, 2013 at 2:16 AM, Lennart Poettering
>> <lennart at poettering.net> wrote:
>> > Then, I generally believe better than trying to be smart when reading
>> > things we should much rather try to place things properly on disk. We
>> > already defrag things based on the read order for btrfs, we should do
>> > the same for ext4. The API for that unfortunately awful, but e4rat has
>> > shown that this does work. Basically, this is what you do:
>>
>> Isn't the problem that reads by blkid and friends are not being caught
>> by readahead-collect, and hence end up blocking until readahead-replay
>> has finished? In that case reordering won't help (I think).
>
> Hmm, if root file system is mounted the file system's superblock should
> be cached in memory, I'd expect, so blkid shouldn't have to block...

I guess it's more about ~30-40 tiny seek()/read() on the main block
device, spread all over the place mainly the first 200 and the last
few kilobyte of the disk, what blkid need to do.

None of of that shares the cache with the mounted filesystem blocks,
and I don't think any of that data is in any other cache at that time.

Kay