[arch-general] pacman too slow on reboot

[Alexander Lam]

On Wed, Feb 10, 2010 at 9:49 AM, Nilesh Govindarajan <lists at itech7.com> wrote:
> Hi,
>
> Recently I'm facing a funny problem. pacman (and even packer) is toooo slow
> to search -Ss (db+aur) or -Qs (db). I see a lot of HDD activity going on.
> But when I run pacman-optimize, it gets fixed.
>
> This happens at every boot. Any ideas ?
>
> --
> Nilesh Govindarajan
> Site & Server Adminstrator
> www.itech7.com
>

Short:
This happens because the pacman db isn't in the cache yet after a
reboot, but after running pacman-optimize, the db is in the cache.

Long:
The cache is unused RAM used to store recent accessed files. When a
file gets read from disk or written out to disk, it is also placed in
the cache, and taken out when more memory is needed for new cache
entries or applications need want more memory.

Since the db is made up of a bunch of small files, this causes the hdd
to have to move its arm around the disk. Linux file systems, unlike
Windows, tries to reduce fragmentation by allocating space for each
file evenly across the disk. The problem with this shows itself with
pacman. The db files are scattered through the disk and the hdd has to
seek (slow operation) to find all of them. So, once all these files
are in cache, the hard drive doesn't need to be accessed to get the
db. Memory is fast at random access (like the hard drive's seek
operation).


> I suggest you look this:
>
> http://bbs.archlinux.org/viewtopic.php?id=20385
>
> It uses one single file to store all pacman's db tree, mounting it via
> loop device, so it gets faster than the native filesystem. I use it
> and the difference is impressive.
>
> HTH.

Thus, with a single file, the Linux file system will try to store it
as a contiguous block on the hard drive, and the hard drive won't have
to seek around, greatly increasing throughput.
-- 
Alexander Lam