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

Consider typical OpenVZ setup, where <code>ext2</code> separate file system is mounted on <code>/vz</code>. <code>ext2</code> is called ''underlying file system'' in such situation. Linux VFS design allows every filesystem file system to export to userspace user space the following information concerning disk space (here and further I use <math>{subscript }</math> is used to specify paricular filesystem 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> filesystem 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 marknote, that the following inequality is always true:

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

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

To produce any calculations , input data are required. What are input data for <code>simfs</code>? Except Aside from already discussed mentioned information from underlying filesystem 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-VE container disk quotas''. The values that provide this element are:

* <math>quota_{used}</math> - the number of blocks currently used by a [[VECT]]* <math>quota_{barrier}</math> - the number of blocks this VE CT can potentially can obtain.

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

<!-- TODO: Uncoment Uncomment after adding examples.

* '''== Cases == Consider three basic possible scenarios. === Quota is off for VE'''CT ===: If quota is off for VE a CT (<code>DISK_QUOTA=no</code>), the total amount of space, that VE this CT potentially can acquire, equals the amount of total space on a partition. Certainly some space can be used by other VEsCTs, but potentially VE a CT can have all the space on device. Number So, the number of free blocks for VE the CT equals the number of free blocks on partition. Note, that it implies that VE a CT root user, can fill in all the space, including the space, that is reserved for root user of HNthe [[host system]]. This is why, you shouldn't one should not reside VEs CTs private areas on root filesystem file system of your HNhost system. Amount The amount of available disk space for VE also CT equals the number of available blocks for the underlying filsystemfile system. Thus, we have the following relationships: 

* ''': Rather valuable disadvantage of switching off OpenVZ quota (besides having unlimited CTs) is that you will not be able to get information about how much disk space is used by a CT (without doing possibly long term <code>du</code> command) using <code>df</code>/<code>stat</code>. I mean, that :: <math>df_{usage} = total_{simfs} - free_{simfs} = total_{ext2} - free_{ext2},</math>: thus in the CT you obtain information about disk usage of partition, but not disk usage of the CT. === Quota is on for VE CT, and there is enough space on partition'''===: [[Image:Vzquota1.png|thumb|left]]: If disk quota is on, the amount of disk space that VE a CT can potentially can acquire should be equal to the quota barrier:

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

: However here is a pitfall. Suppose that the amount of free disk space actually on the underlying filesystem is less than it is estimated from quota using the formule formula above, i.e.::: <math>free_{ext2} < quota_{barrier} - quota_{used} > free_{ext2}</math>: Then, definitely, the amount of free disk space reported by <code>simfs </code> should be other! different. This situation will be considered in the next point and in this point examined later; here we assume that there is enough space on partition, i.e:: <math>free_{ext2} \ge quota_{barrier} - quota_{used} \le free_{ext2}</math> (2): As concerns for amount of disk space available for non-root users, if there is enough disk space::: <math>avail_{ext2} \ge quota_{barrier} - quota_{used} \le avail_{ext2}</math>: and this is right in current point due to assumption (2) and the inequality (1), then amount of disk space available for non-root users in [[VE]] a CT equals the free space estimated from quota:

* '''Quota is on for VE and there is NOT enough space on partition''': 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 VE. Hence::: <math>free_{simfs} = free_{ext2}</math>: And now consider the following situation. There is two VEs. One of VEs writes nothing to disk. Second VE writes to disc some information. Administrator of VE #1 looks at <code>df</code> output. He observes the "Usage" column. What does he see?:: <math>df_{usage} = total_{simfs} - free_{simfs} = total_simfs - free_{ext2}</math> (3): <math>free_{ext2}</math> decreases because [[VE]] #2 writes to disc, consequently <math>df_usage</math> increases! "What the hell is going on?!" - thinks the administrator? - "Nobody writes on the disk in my [[VE]], but the usage increases!" To avoid such situation the following approach is used in OpenVZ disk quota: decrease <math>simfs_total</math> so, that <math>df_usage</math> remains the same, i.e.::: <math>total_{simfs} = total_{ext2} - (quota_{barrier} - quota_{usage})</math> (4): Substituting (4) in (3) obtain::: <math>df_{usage} = (total_{ext2} - free_{ext2}) - (quota_{barrier} - quota_{usage}) = const</math>: In this case, administrator of VE #1 sees that total amount of space decreases, but usage however is constant and it is good.: The same reasoning as with <math>free_simfs</math> suits for calculating <math>avail_simfs</math>. Two cases are possible. If:: <math>quota_{barrier} - quota_{used} \le avail_{ext2}</math>: then :: <math>avail_{simfs} = free_{ext2}</math>: and if:: <math>quota_{barrier} - quota_{used} > avail_{ext2}</math>: then:: <math>avail_{simfs} = quota_{barrier} - quota_{used}</math>

We have three variants. Variant number one is not good, because VE administrator can't get information about VE disk usage. Variant three is not good 'cause we have some weird (but logical) values in <codemath>df</code>/<code>stat</code> output in VE, e.g. total disk space can decrease. Variant two is perfect. How can we provide this varaint always take place? Here is the rule:free_{simfs} = quota_{Out|Do not set random disk quota barrier/limit!}- quota_{used}</math>

Even if you want VE to be unlimited, consider reasonable values. Use the following formula:<math>avail_{simfs} = quota_{limit} - quota_{used}</math>|-:: | <math>free_{ext2} \sum_ge quota_{i=1limit}^Nq_i = S - squota_{used}</math><math>q_iavail_{ext2} \le quota_{limit} - quota_{used}</math> - quota | <math>total_{simfs} = quota_{barrier for VE i}</math>

<math>Sfree_{simfs} = quota_{limit} - quota_{used}</math> - total amount of space on underlying filesystem

<math>savail_{simfs} = avail_{ext2}</math> |- amount of space | <math>free_{ext2} \le quota_{limit} - quota_{used by not VE private data: templates, locks, etc.}</math>'''<math>avail_{ext2} \le quota_{limit} - quota_{used}</math>| <math>total_{simfs} = quota_{usage} + free_{ext2}</math>