Re: [gentoo-user] ext4/ext3 for /boot?

[Stroller <stroller@stellar.eclipse.co.uk>]

On 7 November 2011, at 19:32, Michael Mol wrote:

> On Mon, Nov 7, 2011 at 2:26 PM, Alan McKinnon <alan.mckinnon@gmail.com> wrote:
>> ext2/3/4 are all backwards compatible. ext4 does have a certain feature
>> (I forget what) that once used breaks this compatibility but you are
>> highly, highly unlikely to ever do that on /boot.
> 
> "Extents," I believe. But I don't know exactly what that means, or
> when it comes into play.

It means, as a huge simplification, that ext4 can allocate a file to blocks 1234 - 1256, instead of having to separately allocate blocks 1234, 1235, 1236, 1237, 1238, 1239, 1240, and so on (as ext3 would have had to do).

This fixes ext3's "slow deletes" problem, because only a single entry in the allocation table needs to be removed, instead of many. If you delete a big file (say a 9gig DVD or 40gig blu-ray .iso image file) it's at least an order of magnitude slower on ext3 than it is on ext4.

As I said, this is a huge simplification, and I'm sure there are folks who would take pleasure in explaining how wrong it is, but it's a good enough explanation for a couple of sentences that you can easily grasp. For more details the "Features - Extents" section of ext4's wikipedia page [1] and this other article [2] (these are top hits on Google for "ext4 extents") look pretty good.

Stroller.




[1] http://en.wikipedia.org/wiki/Ext4#Features
[2] http://computer-forensics.sans.org/blog/2011/03/28/digital-forensics-understanding-ext4-part-3-extent-trees