Editing Disk quota, df and stat weird behaviour

The aim of this article is to understand where the numbers that are shown by <code>stat</code>/<code>df</code> utils in [[container]] come from.

== Conventions and notations ==

 

Linux VFS design allows every file system to export to user space the following information concerning disk space (here and further <math>{subscript}</math> is used to specify particular file system type):

Note that not all free blocks can be used by non-root users: some amount of disk space is reserved for root. For example on <code>ext2</code> file system only root can use last free 5 percent (by default)  of disk space. This is the difference between <math>avail_{ext2}</math> and <math>free_{ext2}</math>. Also note, that the following inequality is always true:

Inside a [[container]], a special file system type is used, called <code>simfs</code>. This file system allows to isolate a particular [[CT]] from other CTs. Hence, when <code>df</code> or <code>stat</code> utilities are invoked, they get information from <code>simfs</code>, which exports the following values (by analogy with <code>ext2</code>):

This article is in fact devoted to how simfs file system calculates the values above.  

To produce any calculations, input data are required. What are input data for <code>simfs</code>? Aside from already mentioned information from underlying file system (<math>total_{ext2}</math>, <math>total_{ext2}</math>, <math>total_{ext2}</math>), one more element comes into force in OpenVZ environment. It is OpenVZ ''per-container disk quotas''. The values that provide this element are:

* <math>quota_{used}</math> - the number of blocks currently used by a [[CT]]

* <math>quota_{barrier}</math> - the number of blocks this CT can potentially obtain

OpenVZ disk quota counts the number of blocks currently used by a CT and prevents this number to be greater than the limit/barrier set.

<!-- TODO: Uncomment after adding examples.

 

Consider three basic possible scenarios.

 

=== Quota is off for CT ===

: If quota is off for a CT (<code>DISK_QUOTA=no</code>), the total amount of space that this CT potentially can acquire equals the amount of total space on a partition. Certainly some space can be used by other CTs, but potentially a CT can have all the space on device. So, the number of free blocks for the CT equals the number of free blocks on partition. Note it implies that a CT root user can fill in all the space, including the space reserved for root user of the [[host system]]. This is why one should not reside CTs private areas on root file system of your host system. The amount of available disk space for CT equals the number of available blocks for the underlying file system. Thus, we have the following relationships:

 

 

=== Quota is on for CT, and there is enough space on partition ===

: [[Image:Vzquota1.png|thumb|left]]

: If disk quota is on, the amount of disk space that a CT can potentially acquire should be equal to the quota barrier:  

: The amount of free space in this case should logically be:

: However here is a pitfall. Suppose that the amount of free disk space on the underlying filesystem is less than it is estimated from quota using the formula above, i.e.:

:  Then, definitely, the amount of free disk space reported by <code>simfs</code> should be different. This situation will be examined later; here we assume that there is enough space on partition, i.e

: As for amount of disk space available for non-root users, if there is enough disk space:

: then amount of disk space available for non-root users in a CT equals the free space estimated from quota:

=== Quota is on for CT and there is NOT enough space on partition ===

[[Image:Vzquota4.png|thumb|left]]

This is the most interesting and difficult to explain case. Nevertheless I tried to do it. So, our assumption is that:

: <math>quota_{barrier} - quota_{used} > free_{ext2}</math>

What should be reported as free space in such case? Of course, <math>free_{ext2}</math>! This is the actual amount of space that can be used by a CT. Hence:

: <math>free_{simfs} = free_{ext2}</math>

Now consider the following situation. There are two containers. First CT writes nothing to disk. Second CT writes something to the disk. An administrator of CT #1 looks at <code>df</code> output, noting the "Usage" column. What does she see?

<math>free_{ext2}</math> decreases because CT #2 writes to disk, consequently <math>df_{usage}</math> increases! “What the hell is going on?!”, — thinks the administrator: “Nobody writes to the disk [in my container], but the usage increases”! To avoid such a situation, the following approach is used in OpenVZ: decrease <math>total_{simfs}</math>, so that <math>df_{usage}</math> remains the same, i.e.:

: <math>total_{simfs} = quota_{usage} + free_{ext2}</math> (4)

By substituting (4) to (3), we get:

: <math>df_{usage} = total_{simfs} - free_{simfs} = quota_{usage} + free_{ext2} - free_{ext2} = quota_{usage} = const</math>

In this case, administrator of CT #1 sees that total amount of space decreases, but usage however is constant.

 

The same reason as with <math>free_{simfs}</math> fits for calculating <math>avail_simfs</math>. Two cases are possible. If

: <math>avail_{ext2} \ge quota_{barrier} - quota_{used}</math>

then  

: <math>avail_{simfs} = free_{ext2}</math>

and if

: <math>avail_{ext2} < quota_{barrier} - quota_{used}</math>

then

: <math>avail_{simfs} = quota_{barrier} - quota_{used}</math>

So there are three basic variants. Variant number one is not good, because a container's administrator can not get information about CT disk usage and the [[host system]] administrator can't limit CT disk usage. Variant three is not good because we have some weird (but logical) values in <code>df</code>/<code>stat</code> output in CT, e.g. total disk space can decrease.  Variant two is perfect. How can we make sure that this variant always take place? Here is the simple rule:

{{Warning|Do not set random disk quota barrier/limit!}}

Even if you want a container to be unlimited, consider reasonable values. Use the following formula:

:: <math>\sum_{i=1}^Nq_i \le S - s</math> (5)

<math>s</math> — amount of space used by everything else than CT private area: templates, locks, etc.

Note that if you install a template, you decrease <math>s</math>. This is bad because, ideally, after each template

installation you have to check inequality (5). To avoid this I suggest to mount separate partition on <code>/vz/private</code>, rather than on <code>/vz</code>. In such case <math>s</math> always equals <math>0</math>.

{| class="wikitable" align="center"

| <math>total_{simfs} = quota_{usage} + free_{ext2}</math>

At the moment I see only two more reasons why numbers in <code>df</code>/<code>stat</code> output can confuse you.

# The quota is inconsistent. This can happen if you turned quota off for some time, if you wrote directly to private area (<code>/vz/private</code>), but not through <code>simfs</code>, etc. When you have doubts whether your quota is consistent or not, just drop quota (<code>vzquota drop <ctid></code>, where <code><ctid></code> is the id of a stopped [[CT]]). While starting [[CT]], <code>vzctl</code> will automatically initialize quota.

# Unsupported underlying filesystem. Currently OpenVZ quota only supports <code>ext2</code>, <code>ext3</code> and <code>ext4</code>. With other file system types you can have unpredictable results. Praemonitus praemunitus!