tmpfs limits?

quayasil · 2012-11-21 16:17:06

One host:

# df -h
Filesystem      Size  Used Avail Use% Mounted on
rootfs           79G   37G   39G  49% /
dev             1.9G     0  1.9G   0% /dev
run             1.9G  404K  1.9G   1% /run
/dev/sda2        79G   37G   39G  49% /
tmpfs           1.9G     0  1.9G   0% /dev/shm
tmpfs           1.9G     0  1.9G   0% /sys/fs/cgroup
tmpfs           1.9G  317M  1.6G  17% /tmp

Other host:

# df -h
Filesystem      Size  Used Avail Use% Mounted on
rootfs          823G  1.5G  780G   1% /
dev              30G     0   30G   0% /dev
run              30G  372K   30G   1% /run
/dev/sda1       823G  1.5G  780G   1% /
tmpfs            30G     0   30G   0% /dev/shm
tmpfs            30G     0   30G   0% /sys/fs/cgroup
tmpfs           2.0G  480K  2.0G   1% /tmp

I suppose it's a systemd related question: why are all those pseudofilesystems of the size 1.9G and 30G ? Can I limit them in size (for potential prevention of DDoS out-of-resources attacks)?

Awebb · 2012-11-21 16:21:35

The size of a tmpfs is usually half your RAM size by default. Those 30G seem strange. You can set the size with the size= mount option.

WonderWoofy · 2012-11-21 16:21:37

I think you need to supply more information. No one can tell you why your filesystems are of different sizes with no information about your systems.

Last edited by WonderWoofy (2012-11-21 16:21:53)

quayasil · 2012-11-21 16:31:31

Awebb wrote:

The size of a tmpfs is usually half your RAM size by default. Those 30G seem strange. You can set the size with the size= mount option.

Thanks. The 30G is OK:

# free -h
             total       used       free     shared    buffers     cached
Mem:           58G       700M        58G         0B        38M       179M
-/+ buffers/cache:       482M        58G
Swap:           0B         0B         0B

Awebb wrote:

You can set the size with the size= mount option.

Beg your pardon: what mount option? Those filesystems are systemd-managed, no fstab entries...

quayasil · 2012-11-21 16:32:18

WonderWoofy wrote:

I think you need to supply more information. No one can tell you why your filesystems are of different sizes with no information about your systems.

I'm not asking about filesystems. Those or systemd-managed pseudofilesystems ;-)

WonderWoofy · 2012-11-21 16:35:43

Yeah, that is what I meant. In other words, you are asking about the sizes of tmpfs without telling us how much ram you have. This is an important factor as Awebb pointed out.

WorMzy · 2012-11-21 17:04:06

quayasil wrote:

Beg your pardon: what mount option? Those filesystems are systemd-managed, no fstab entries...

You can still create one.

$ grep '/tmp' /etc/fstab 
tmpfs   /tmp   tmpfs   nodev,nosuid   0   0

$ systemctl status tmp.mount 
tmp.mount - /tmp
	  Loaded: loaded (/etc/fstab; static)
	  Active: active (mounted) since Wed, 2012-11-21 11:24:01 GMT; 5h 36min ago
	   Where: /tmp
	    What: tmpfs
	 Process: 272 ExecMount=/bin/mount tmpfs /tmp -t tmpfs -o nodev,nosuid (code=exited, status=0/SUCCESS)
	  CGroup: name=systemd:/system/tmp.mount

Nov 21 11:24:01 sakura systemd[1]: Mounted /tmp.

Last edited by WorMzy (2012-11-21 17:04:26)

quayasil · 2012-11-21 17:08:55

What is the recommended place for it: /etc/fstab or /usr/lib/systemd/system/tmp.mount or any other file?

Last edited by quayasil (2012-11-21 17:09:44)

WorMzy · 2012-11-21 18:27:43

Never modify anything in /usr/lib/systemd, as these changes will be overwritten when you update/downgrade/reinstall the package that provides the file you've modified.

If you want to modify the .mount files, copy them from /usr/lib/systemd/system to /etc/systemd/system, and then edit them there. The /etc copies will be used instead of the /usr equivalents.

As for *.mount files vs fstab, it all comes down to user preference.

quayasil · 2012-11-21 18:43:07

Yes... and no. By editing any of those files I can set limit on /tmp , for example to 200MB :

# df -h
Filesystem      Size  Used Avail Use% Mounted on
rootfs           79G   37G   39G  49% /
dev             1.9G     0  1.9G   0% /dev
run             1.9G  404K  1.9G   1% /run
/dev/sda2        79G   37G   39G  49% /
tmpfs           1.9G     0  1.9G   0% /dev/shm
tmpfs           1.9G     0  1.9G   0% /sys/fs/cgroup
tmpfs           200M  317M  1.6G  17% /tmp

It's not exactly all I tried to do. Other tmpfses are 2GB (half of my RAM). How about, for example: /sys/fs/cgroup ? I don't think setting an entry for this in /etc/fstab is a good idea.

# grep -R cgroup /usr/lib/systemd/system

gives no matches. So where is it? Still I suspect the systemd but where?

65kid · 2012-11-21 18:52:55

quayasil wrote:

Yes... and no. By editing any of those files I can set limit on /tmp , for example to 200MB :
# df -h
Filesystem      Size  Used Avail Use% Mounted on
rootfs           79G   37G   39G  49% /
dev             1.9G     0  1.9G   0% /dev
run             1.9G  404K  1.9G   1% /run
/dev/sda2        79G   37G   39G  49% /
tmpfs           1.9G     0  1.9G   0% /dev/shm
tmpfs           1.9G     0  1.9G   0% /sys/fs/cgroup
tmpfs           200M  317M  1.6G  17% /tmp
It's not exactly all I tried to do. Other tmpfses are 2GB (half of my RAM). How about, for example: /sys/fs/cgroup ? I don't think setting an entry for this in /etc/fstab is a good idea.
# grep -R cgroup /usr/lib/systemd/system
gives no matches. So where is it? Still I suspect the systemd but where?

afaik the cgroup mounts are hardcoded into systemd as they are required for systemd to work properly. As you can see they don't actually use space, so I wouldn't worry about them.

btw, systemd will mount /tmp by default, even without an fstab entry (falconindy said that the /tmp entry in the default fstab will be removed at some point since it is redundant). If you want to override the /tmp settings, use fstab. Overriding tmp.mount will also work, but it is more complex and even upstream recommends using fstab.

quayasil · 2012-11-21 18:58:46

65kid wrote:

afaik the cgroup mounts are hardcoded into systemd as they are required for systemd to work properly. As you can see they don't actually use space, so I wouldn't worry about them.

Sounds like an Apple-like "kind of magic but don't worry" I'm afraid there is growing contradiction between systemd and the KISS principle

WorMzy · 2012-11-21 19:18:58

quayasil wrote:

How about, for example: /sys/fs/cgroup ? I don't think setting an entry for this in /etc/fstab is a good idea.

I don't see why you'd think setting it in a mount unit would be better or worse than setting it in fstab. At the end of the day, they both do the same thing.

Any way, as a proof of concept, I made an entry in my fstab for that tmpfs:

$ grep /sys/fs/cgroup /etc/fstab 
tmpfs /sys/fs/cgroup tmpfs rw,nosuid,nodev,noexec,mode=755,size=200M 0 0

$ mount | grep "/sys/fs/cgroup " 
tmpfs on /sys/fs/cgroup type tmpfs (rw,nosuid,nodev,noexec,relatime,size=204800k,mode=755)

$ /bin/df -h /sys/fs/cgroup 
Filesystem      Size  Used Avail Use% Mounted on
tmpfs           200M     0  200M   0% /sys/fs/cgroup

As you can see, it worked fine.

I don't see why you're so worried about these tmpfs' in the first place. Unless you're planning on moving a 2TB file into one of them or something, you really don't need to worry about them.

Arch Linux

#1 2012-11-21 16:17:06

tmpfs limits?

#2 2012-11-21 16:21:35

Re: tmpfs limits?

#3 2012-11-21 16:21:37

Re: tmpfs limits?

#4 2012-11-21 16:31:31

Re: tmpfs limits?

#5 2012-11-21 16:32:18

Re: tmpfs limits?

#6 2012-11-21 16:35:43

Re: tmpfs limits?

#7 2012-11-21 17:04:06

Re: tmpfs limits?

#8 2012-11-21 17:08:55

Re: tmpfs limits?

#9 2012-11-21 18:27:43

Re: tmpfs limits?

#10 2012-11-21 18:43:07

Re: tmpfs limits?

#11 2012-11-21 18:52:55

Re: tmpfs limits?

#12 2012-11-21 18:58:46

Re: tmpfs limits?

#13 2012-11-21 19:18:58

Re: tmpfs limits?

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