Re: [RFC/PATCH 0/2] ext4: Transparent Decompression Support

From: Dhaval Giani
Date: Thu Jul 25 2013 - 15:27:24 EST

On 2013-07-25 1:53 PM, JÃrn Engel wrote:
On Thu, 25 July 2013 09:42:18 -0700, Taras Glek wrote:
Footprint wins are useful on android, but it's the
increased IO throughput on crappy storage devices that makes this
most attractive.
All the world used to be a PC. Seems to be Android these days.

The biggest problem with compression support in the past was the
physical properties of hard drives (the spinning type, if you can
still remember those). A random seek is surprisingly expensive, of a
similar cost to 1MB or more of linear read. So anything that
introduces more random seeks will kill the preciously little
performance you had to begin with.

As long as files are write-once and read-only from that point on, you
can just append a bunch of compressed chunks on the disk and nothing
bad happens. But if you have a read-write file with random overwrites
somewhere in the middle, those overwrites will change the size of the
compressed data. You have to free the old physical blocks on disk and
allocate new ones. In effect, you have auto-fragmentation.

So if you want any kind of support for your approach, I suspect you
should either limit it to write-once files or prepare for a mob of
gray-haired oldtimers with rainbow suspenders complaining about
performance on their antiquated hardware. And the mob may be larger
than you think.

Yes, we plan to limit it to write-once. In order to write, you have to
replace the file.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at