Editing Disk quota, df and stat weird behaviour
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− | The aim of this article is to understand where the numbers that are shown by <code>stat</code>/<code>df</code> utils in [[ | + | The aim of this article is to understand where the numbers that are shown by <code>stat</code>/<code>df</code> utils in [[VE]] come from. |
− | == Conventions and | + | == Conventions and Notations == |
+ | Consider typical OpenVZ setup, where <code>ext2</code> separate filesystem is mounted on <code>/vz</code>. <code>ext2</code> is called ''underlying filesystem'' in such situation. | ||
− | + | Linux VFS design allows every filesystem to export to userspace the following information concerning disk space (here and further I use subscript to specify paricular filesystem type): | |
− | |||
− | Linux VFS design allows every | ||
* <math>total_{ext2}</math> - total amount of disk space that potentially can be acquired (e.g. HDD capacity) | * <math>total_{ext2}</math> - total amount of disk space that potentially can be acquired (e.g. HDD capacity) | ||
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* <math>avail_{ext2}</math> - amount of disk space that is still available for non-root users | * <math>avail_{ext2}</math> - amount of disk space that is still available for non-root users | ||
− | 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> | + | 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 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 mark, that the following inequality is always true: |
: <math>avail_{ext2} \le free_{ext2}</math> (1) | : <math>avail_{ext2} \le free_{ext2}</math> (1) | ||
− | Inside | + | Inside [[VE]] special filesystem type is used: <code>simfs</code>. This filesystem allows to isolate particular [[VE]] from other [[VE]]s. Hence, when <code>df</code> or <code>stat</code> utils are invoked they get information from <code>simfs</code>, which exports to them the following values (by analogy with <code>ext2</code>): |
* <math>total_{simfs}</math> | * <math>total_{simfs}</math> | ||
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* <math>avail_{simfs}</math> | * <math>avail_{simfs}</math> | ||
− | This article is in fact devoted to how simfs | + | This article is in fact devoted to how simfs filesystem calculates the values above. |
− | To produce any calculations | + | To produce any calculations input data are required. What are input data for <code>simfs</code>? Except already discussed information from underlying filesystem (<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 disk quotas''. The values that provide this element are: |
− | * <math>quota_{used}</math> - the number of blocks currently used by | + | * <math>quota_{used}</math> - the number of blocks currently used by [[VE]] |
− | * <math>quota_{barrier}</math> - the number of blocks this | + | * <math>quota_{barrier}</math> - the number of blocks this [[VE]] potentially can obtain |
− | OpenVZ disk quota counts the number of blocks currently used by | + | OpenVZ disk quota counts the number of blocks currently used by VE and prevents this number to be greater than the limit/barrier set. |
− | <!-- TODO: | + | <!-- TODO: Uncoment after adding examples. |
First let's use | First let's use | ||
<pre> | <pre> | ||
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== Cases == | == Cases == | ||
− | + | Consider three basic scenarios, that are possible. | |
− | Consider three basic possible | + | === '''Quota is off for VE''' === |
− | + | : If quota is off for [[VE]] (DISK_QUOTA=no), the total amount of space, that [[VE]] potentially can acquire, equals amount of total space on partition. Certainly some space can be used by other [[VE]]s, but potentially [[VE]] can have all the space on device. Number of free blocks for [[VE]] equals number of free blocks on partition. Note, that it implies that [[VE]] root user, can fill all the space, including the space, that is reserved for root user of [[HN]]. This is why, you shouldn't reside [[VE]]s private areas on root filesystem of your [[HN]]. Amount of available disk space for [[VE]] equals the number of available blocks for underlying filsystem. Thus, we have the following relationships: | |
− | === Quota is off for | ||
− | : If quota is off for | ||
− | |||
:: <math>total_{simfs}</math> = <math>total_{ext2}</math> | :: <math>total_{simfs}</math> = <math>total_{ext2}</math> | ||
:: <math>free_{simfs}</math> = <math>free_{ext2}</math> | :: <math>free_{simfs}</math> = <math>free_{ext2}</math> | ||
:: <math>avail_{simfs} = avail_{ext2}</math> | :: <math>avail_{simfs} = avail_{ext2}</math> | ||
+ | : Rather valuable disadvantage of swithching OpenVZ quota off (besides having unlimited [[VE]]s!) is that you will not be able to get information about how much disk space is used by [[VE]] (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 [[VE]] you obtain information about disk usage of partition, but not disk usage of [[VE]]. | ||
− | + | === '''Quota is on for VE and there is enough space on partition''' ( [[:Image:Vzquota1.png|illustration 1]]) === | |
− | + | : If quota is on, amount of disk space that [[VE]] potentially can acquire should be equal quota barrier: | |
− | |||
− | |||
− | === Quota is on for | ||
− | |||
− | : If | ||
:: <math>total_{simfs} = quota_{barrier}</math> | :: <math>total_{simfs} = quota_{barrier}</math> | ||
− | : | + | : Amount of free space in this case should logically be the following: |
:: <math>free_{simfs} = quota_{barrier} - quota_{used}</math> | :: <math>free_{simfs} = quota_{barrier} - quota_{used}</math> | ||
− | : However here is a pitfall. Suppose that the amount of free disk space on | + | : However here is a pitfall. Suppose that the amount of free disk space actually on underlying filesystem is less than it is estimated from quota using the formule above, i.e.: |
:: <math> free_{ext2} < quota_{barrier} - quota_{used} </math> | :: <math> free_{ext2} < quota_{barrier} - quota_{used} </math> | ||
− | : Then, definitely, | + | : Then, definitely, amount of free disk space reported by <code>simfs</code> should be other! This situation will be considered in the next point and in this point we assume that there is enough space on partition, i.e |
:: <math> free_{ext2} \ge quota_{barrier} - quota_{used} </math> (2) | :: <math> free_{ext2} \ge quota_{barrier} - quota_{used} </math> (2) | ||
− | : As | + | : As concerns amount of disk space available for non-root users, if there is enough disk space: |
:: <math>avail_{ext2} \ge quota_{barrier} - quota_{used}</math> | :: <math>avail_{ext2} \ge quota_{barrier} - quota_{used}</math> | ||
− | : then amount of disk space available for non-root users in | + | : then amount of disk space available for non-root users in [[VE]] equals free space estimated from quota: |
:: <math>free_{simfs} = quota_{barrier} - quota_{used}</math> | :: <math>free_{simfs} = quota_{barrier} - quota_{used}</math> | ||
− | |||
− | === Quota is on for | + | === '''Quota is on for VE and there is NOT enough space on partition''' ([[:Image:Vzquota2.png|illustration 2]], [[:Image:Vzquota3.png|illustration 3]], [[:Image:Vzquota4.png|illustration 4]], [[:Image:Vzquota5.png|illustration 5]]) === |
− | + | : This is the most interesting and difficult to explain case. Nevertheless I tried to do it. So, our assumption is that: | |
− | [[Image:Vzquota2.png| | + | :: <math>quota_{barrier} - quota_{used} > free_{ext2}</math> |
− | [[Image:Vzquota3.png| | + | : 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: |
− | [[Image:Vzquota4.png| | + | :: <math>free_{simfs} = free_{ext2}</math> |
− | [[Image:Vzquota5.png| | + | : And now consider the following situation. There is two [[VE]]s. One of [[VE]]s 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 she see? |
− | |||
− | 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 | ||
− | : <math>free_{simfs} = free_{ext2}</math> | ||
− | |||
:: <math>df_{usage} = total_{simfs} - free_{simfs} = total_{simfs} - free_{ext2}</math> (3) | :: <math>df_{usage} = total_{simfs} - free_{simfs} = total_{simfs} - free_{ext2}</math> (3) | ||
− | <math>free_{ext2}</math> decreases because | + | : <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: 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) | + | :: <math>total_{simfs} = quota_{usage} + free_{ext2}</math> (4) |
− | + | : Substituting (4) in (3) obtain: | |
− | : <math>df_{usage} = total_{simfs} - free_{simfs} = quota_{usage} + free_{ext2} - free_{ext2} = quota_{usage} = const</math> | + | :: <math>df_{usage} = total_{simfs} - free_{simfs} = quota_{usage} + free_{ext2} - free_{ext2} = quota_{usage} = const</math> |
− | In this case, administrator of | + | : In this case, administrator of [[VE]] #1 sees that total amount of space decreases, but usage however is constant. |
− | + | : The same reasoning as with <math>free_{simfs}</math> suits for calculating <math>avail_simfs</math>. Two cases are possible. If | |
− | The same | + | :: <math>avail_{ext2} \ge quota_{barrier} - quota_{used}</math> |
− | : <math>avail_{ext2} \ge quota_{barrier} - quota_{used}</math> | + | : then |
− | then | + | :: <math>avail_{simfs} = free_{ext2}</math> |
− | : <math>avail_{simfs} = free_{ext2}</math> | + | : and if |
− | and if | + | :: <math>avail_{ext2} < quota_{barrier} - quota_{used}</math> |
− | : <math>avail_{ext2} < quota_{barrier} - quota_{used}</math> | + | : then |
− | then | + | :: <math>avail_{simfs} = quota_{barrier} - quota_{used}</math> |
− | : <math>avail_{simfs} = quota_{barrier} - quota_{used}</math> | ||
The table below summarizes all possible cases. | The table below summarizes all possible cases. | ||
− | |||
− | |||
== Cases Conclusion == | == Cases Conclusion == | ||
− | So | + | So we have three basic variants. Variant number one is not good, because [[VE]] administrator can't get information about [[VE]] disk usage and [[HN]] administrator can't limit [[VE]] disk usage. Variant three is not good 'cause we have some weird (but logical) values in <code>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 simple rule: |
− | {{ | + | {{Out|Do not set random disk quota barrier/limit!}} |
− | Even if you want | + | Even if you want [[VE]] to be unlimited, consider reasonable values. Use the following formula: |
+ | :: <math>\sum_{i=1}^Nq_i \le S - s</math> (5) | ||
+ | <math>q_i</math> - quota barrier for [[VE]] <math>i</math> | ||
− | + | <math>S</math> - total amount of space on underlying filesystem | |
− | + | <math>s</math> - amount of space used by not [[VE]]s private area: templates, locks, etc. | |
− | |||
− | <math>s</math> | ||
− | Note that if you install | + | Note, that if you install 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 | + | installation you have to check inequality (5). To avoid this I suggest to mount separate partion on /vz/private, rather than |
+ | on /vz/. In such case <math>s</math> always equals <math>0</math>. | ||
== Cases Summarizing Table == | == Cases Summarizing Table == | ||
− | {| | + | {| border="1" cellpadding="5" cellspacing="0" align="center" |
| colspan="2" | Quota off | | colspan="2" | Quota off | ||
| <math>total_{simfs} = total_{ext2}</math> | | <math>total_{simfs} = total_{ext2}</math> | ||
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== Other reasons of strange numbers == | == Other reasons of strange numbers == | ||
− | At the moment I see only two more reasons why numbers in <code>df</code>/<code>stat</code> output can confuse you. | + | 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 <veid></code>, where <code><veid></code> is the id of stopped [[VE]]). While starting [[VE]] <code>vzctl</code> will automatically initalize quota. | |
− | + | * Unsupported underlying filesystem. Currently OpenVZ quota only supports <code>ext2</code> and <code>ext3</code>. With other filesystem types you can have unpredictable results. Praemonitus praemunitus! | |
== TODO == | == TODO == | ||
TODO: Add examples with stat/df | TODO: Add examples with stat/df | ||
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