Ext3/ext4 reserved blocks

[Edward_Iii]

Hello all,

I recently did a fresh install of openSUSE 11.3 without formatting my home partition. When it was done I installed some applications and decided to clean up my home partition.

I began with ~35GiB out of ~240GiB and when I was finished it was ~27GiB.
But when I tried to find the source I discovered that my home partition was only filled with ~15GiB of files.
So where did the other 12GiB gone?

After some searching I discovered that every ext2/ext3/ext4 partition reserves 5% of the partition size by default. (240GiB / 100 * 5 = 12GiB Found them!)
You can easily reduce this amount with this command:

tune2fs -r 0 /dev/sdxX

I've still got some questions left:
Where is this space needed for?, I've read things like the:

• System uses it as some sort swap.
• It is used for defragmentation.

Is this space needed on a data partition? (If it's for defragmentation the answer would be no, but recommended)
Is 5% really needed? (For partition greater than 512GiB the reserved space is way too much, ~25,6GiB)

I'd also like to hear what you've done/heard about reserved blocks.

Thanks in advance!

[DaaX]

Wow! This is really interesting and never heard of that and never noticed that. I'll go see right away.

I am eager to know why is that.

[Edward_Iii]

Hello DaaX,

Wow! This is really interesting and never heard of that and never noticed that. I'll go see right away.

I am eager to know why is that.

Still not sure where it's exactly for but I found this:

If you set the reserved block count to zero, it won't affect
performance much except if you run for long periods of time (with lots
of file creates and deletes) while the filesystem is almost full
(i.e., say above 95%), at which point you'll be subject to
fragmentation problems. Ext4's multi-block allocator is much more
fragmentation resistant, because it tries much harder to find
contiguous blocks, so even if you don't enable the other ext4
features, you'll see better results simply mounting an ext3 filesystem
using ext4 before the filesystem gets completely full.

If you are just using the filesystem for long-term archive, where
files aren't changing very often (i.e., a huge mp3 or video store), it
obviously won't matter.

Source: Reserved block count for Large Filesystem - Linux Archive

Still searching for more information!

[DenverD]

linux (in face, all *nix) goes crazy if it runs out of space...among
other things you can't even log in to fix the out of space problem
_until_ there is space..

the 5% is reserved for roots use only....so root can log in and fix
the out of space problem..

sure, you can reduce below 5%, but why? drives space is cheap, get
more...and KEEP your 5% insurance that you won't lock yourself out..


--
DenverD
CAVEAT: http://is.gd/bpoMD
[NNTP posted w/openSUSE 11.3, KDE4.5.5, Thunderbird3.0.11, nVidia
173.14.28 3D, Athlon 64 3000+]
"It is far easier to read, understand and follow the instructions than
to undo the problems caused by not." DD 23 Jan 11

[hcvv]

This is espcialy important for what you could call "system disks". Just take the root partition / as the most important example for us normal Linux PC users. When this partition becomes "full", the users can not create any file anymore, But all root processes still can in that "reserved space". This means that e.g. logging and other important processes can cary on and thus the system will not break down immediatly. The system manager then has a bit of time to search for the problem and cure it (by e.g. removing ol log files, but it depends on what the problem is).

For "user only"" file systems (like /home) this is not an important argument. But it could be argued that on a file system that is on the brink of being full, only scattered small pieces of disk are available and thus new files will be spread all over the disk and thus can be bad for performace. On the other hand, with todays large disks, 5% may be a lot of GBs.
Thus one could argue setting this to e.g. 1% on big user only file systems. Or even to 0%.

[ken_yap]

It's for performance reasons as Tso has explained.

Bear in mind that while X GB or whatever sounds like a lot of space, by the time you get to 95% full, you'll be looking for new disk drive anyway and that extra space gained will only buy you a month or two more of usage which at the costs of disk drives these days, will only be a few bucks worth.

But of course by all means lower the reserved block percentage on seldom changing partitions.

[Edward_Iii]

Hello hcvv and ken_yap,

Thanks for your explanation!

But of course by all means lower the reserved block percentage on seldom changing partitions.

Lowered it on some of my data partitions.

Thanks again!

[DaaX]

Hi hcvv and ken_yap,

I also thank you for the explanations. I never thought about that. Linux is really a great system and well thought for that matter.

[hcvv]

Hi hcvv and ken_yap,

I also thank you for the explanations. I never thought about that. Linux is really a great system and well thought for that matter.

Let us give credit to who earns it. It is not a Linux invention. It was in Unix long before.