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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 [[container]] come from. | + | 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 notations ==
| + | Consider typical OpenVZ setup, where ext2 filesystem is mounted on /vz. So underlying filesystem in this situation is ext2. |
| | | | |
| − | 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 simfs (root file system type for VE) to get the following information concerning disk space from underlying filesystem: |
| | | | |
| − | 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):
| + | * '''ext2_total''' - total amount of disk space that potentially can be acquired |
| | + | * '''ext2_free''' - amount of disk space that is still free |
| | + | * '''ext2_avail''' - amount of disk space that is still available for non-root users |
| | | | |
| − | * <math>total_{ext2}</math> - total amount of disk space that potentially can be acquired (e.g. HDD capacity)
| + | Note, that not all free blocks can be used by non-root users. By default ext2 sets 5% barrier: 5 percent of total amount of disk space is reserved for super user. This is the difference between '''ext2_avail''' and '''ext2_free'''. Note, that the following inequality is always true: |
| − | * <math>free_{ext2}</math> - amount of disk space that is still free
| + | : '''ext2_avail ≤ ext2_free (1)''' |
| − | * <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> 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:
| + | Absolutely the same set of information about disk space simfs should export to user-space if it is asked about it (e.g. <code>stat</code> or <code>df</code> command were invoked): |
| − | : <math>avail_{ext2} \le free_{ext2}</math> (1)
| |
| | | | |
| − | 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>):
| + | * '''simfs_total''' |
| | + | * '''simfs_free''' |
| | + | * '''simfs_avail''' |
| | | | |
| − | * <math>total_{simfs}</math>
| + | In OpenVZ environment one more element is pertinent in the situation we're considering: OpenVZ disk quota. First level of OpenVZ disk quota counts the number of blocks currently used by VE ('''q_used''') and prevents this number to be greater than the limit/barrier set ('''q_barrier'''). |
| − | * <math>free_{simfs}</math>
| |
| − | * <math>avail_{simfs}</math>
| |
| | | | |
| − | This article is in fact devoted to how simfs file system calculates the values above.
| + | <!-- TODO: Uncoment after adding examples. |
| − | | |
| − | 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. | |
| | First let's use | | First let's use |
| | <pre> | | <pre> |
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| | command to get information we need, and later discuss df output. | | command to get information we need, and later discuss df output. |
| | --> | | --> |
| | + | Consider three basic scenarios, that are possible: |
| | | | |
| − | == Cases ==
| + | * '''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 VEs, 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 VEs private areas on root filesystem of your HN. Amount of available disk space for VE also equals the number of available blocks for underlying filsystem. Thus, we have the following relationships: |
| − | Consider three basic possible scenarios.
| + | :: '''simfs_total = ext2_total''' |
| − | | + | :: '''simfs_free = ext2_free''' |
| − | === Quota is off for CT ===
| + | :: '''simfs_avail = ext2_avail''' |
| − | : 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: | + | : Rather valueable disadvantage of swithching OpenVZ quota off 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). I mean, that |
| − | | + | :: '''df_usage = simfs_total - simfs_free = ext2_total - ext2_free''' |
| − | :: <math>total_{simfs}</math> = <math>total_{ext2}</math> | + | : thus in VE you obtain information about disk usage of partition, but not disk usage of VE. By the way, just this number is displayed by <code>df</code> tool in "Usage" column. |
| − | :: <math>free_{simfs}</math> = <math>free_{ext2}</math> | |
| − | :: <math>avail_{simfs} = avail_{ext2}</math> | |
| − | | |
| − | : 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 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:
| |
| − | :: <math>total_{simfs} = quota_{barrier}</math>
| |
| − | : The amount of free space in this case should logically be:
| |
| − | :: <math>free_{simfs} = quota_{barrier} - quota_{used}</math>
| |
| − | : 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.:
| |
| − | :: <math> free_{ext2} < quota_{barrier} - quota_{used} </math>
| |
| − | : 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
| |
| − | :: <math> free_{ext2} \ge quota_{barrier} - quota_{used} </math> (2)
| |
| − | : As for amount of disk space available for non-root users, if there is enough disk space:
| |
| − | :: <math>avail_{ext2} \ge quota_{barrier} - quota_{used}</math>
| |
| − | : then amount of disk space available for non-root users in a CT equals the free space estimated from quota:
| |
| − | :: <math>free_{simfs} = quota_{barrier} - quota_{used}</math>
| |
| − | {{Clear}}
| |
| − | | |
| − | === Quota is on for CT and there is NOT enough space on partition ===
| |
| − | {|
| |
| − | [[Image:Vzquota2.png|thumb|left]]
| |
| − | [[Image:Vzquota3.png|thumb|left]]
| |
| − | [[Image:Vzquota4.png|thumb|left]]
| |
| − | [[Image:Vzquota5.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>df_{usage} = total_{simfs} - free_{simfs} = total_{simfs} - free_{ext2}</math> (3)
| |
| − | <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>
| |
| − | | |
| − | The table below summarizes all possible cases.
| |
| − | |}
| |
| − | {{Clear}}
| |
| − | | |
| − | == Cases Conclusion ==
| |
| − | 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)
| |
| − | | |
| − | Here <math>q_i</math> is quota barrier for CT<math>i</math>,<br/>
| |
| − | <math>S</math> — total amount of space on underlying file system<br/>
| |
| − | <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>.
| |
| − | | |
| − | == Cases Summarizing Table ==
| |
| − | {| class="wikitable" align="center"
| |
| − | | colspan="2" | Quota off
| |
| − | | <math>total_{simfs} = total_{ext2}</math>
| |
| | | | |
| − | <math>free_{simfs} = free_{ext2}</math>
| + | * '''Quota is on for VE and there is enough space on partition''' |
| | + | : If quota is on, amount of disk space that VE potentially can acquire should be equal quota barrier: |
| | + | :: '''simfs_total = q_barrier''' |
| | + | : Amount of free space in this case should logically be the following: |
| | + | :: '''simfs_free = q_barrier - q_used''' |
| | + | : However here is a pitfall. Suppose that amount of free disk space actually on underlying filesystem is less than it is estimated from quota using the formule above, i.e.: |
| | + | :: '''q_barrier - q_used > ext2_free''' |
| | + | : Then, definitely, amount of free disk space reported by simfs 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 |
| | + | :: '''q_barrier - q_used ≤ ext2_free (2)''' |
| | + | : As concerns amount of disk space available for non-root users, if there is enough disk space: |
| | + | :: '''q_barrier - q_used ≤ ext2_avail''' |
| | + | : 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 equals free space estimated from quota: |
| | + | :: '''simfs_free = q_barrier - q_used''' |
| | | | |
| − | <math>avail_{simfs} = avail_{ext2}</math> | + | * '''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: |
| − | | rowspan = 3 | Quota on
| + | :: '''q_barrier - q_used > ext2_free''' |
| − | | <math>free_{ext2} \ge quota_{limit} - quota_{used}</math>
| + | : What should be reported as free space in such case? Of course, '''ext2_free'''! This is the actual amount of space that can be used by VE. Hence: |
| − | <math>avail_{ext2} \ge quota_{limit} - quota_{used}</math>
| + | :: '''simfs_free = ext2_free''' |
| − | | <math>total_{simfs} = quota_{barrier}</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? |
| | + | :: '''df_usage = simfs_total - simfs_free = simfs_total - ext2_free (3)''' |
| | + | : '''ext2_free''' decreases because VE #2 writes to disc, consequently '''df_usage''' 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 '''simfs_total''' so, that '''df_usage''' remains the same, i.e.: |
| | + | :: '''simfs_total = ext2_total - (q_barrier - q_usage) (4)''' |
| | + | : Substituting (4) in (3) obtain: |
| | + | :: '''df_usage = (ext2_total - ext2_free) - (q_barrier - q_usage) = const''' |
| | + | : 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 '''simfs_free''' suits for calculating '''simfs_avail'''. Two cases are possible. If |
| | + | :: '''q_barrier - q_used ≤ ext2_avail''' |
| | + | : then |
| | + | :: '''simfs_avail = ext2_free''' |
| | + | : and if |
| | + | :: '''q_barrier - q_used > ext2_avail''' |
| | + | : then |
| | + | :: '''simfs_avail = q_barrier - q_used''' |
| | | | |
| − | <math>free_{simfs} = quota_{limit} - quota_{used}</math>
| + | The table below summarize all possible cases. |
| | | | |
| − | <math>avail_{simfs} = quota_{limit} - 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 <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 rule: |
| − | |-
| + | * '''Do not set random disk quota barrier/limit! Even if you want VE to be unlimited, consider reasonable values. Use the following fromule |
| − | | <math>free_{ext2} \ge quota_{limit} - quota_{used}</math>
| + | :: <math>\sum_{i=1}^Nq_i = S - s</math> |
| − | <math>avail_{ext2} \le quota_{limit} - quota_{used}</math> | + | <math>q_i</math> - quota barrier for VE i |
| − | | <math>total_{simfs} = quota_{barrier}</math>
| |
| | | | |
| − | <math>free_{simfs} = quota_{limit} - quota_{used}</math> | + | <math>S</math> - total amount of space on underlying filesystem |
| | | | |
| − | <math>avail_{simfs} = avail_{ext2}</math> | + | <math>s</math> - amount of space used by not VE private data: templates, locks, etc. |
| − | |-
| + | ''' |
| − | | <math>free_{ext2} \le quota_{limit} - quota_{used}</math>
| |
| − | <math>avail_{ext2} \le quota_{limit} - quota_{used}</math>
| |
| − | | <math>total_{simfs} = quota_{usage} + free_{ext2}</math>
| |
| | | | |
| − | <math>free_{simfs} = free_{ext2}</math>
| |
| | | | |
| − | <math>avail_{simfs} = avail_{ext2}</math>
| + | ----- |
| − | |}
| + | TODO: Add Roma's images |
| | | | |
| − | == Other reasons of strange numbers ==
| + | TODO: Add table |
| − | 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!
| |
| | | | |
| − | == TODO ==
| |
| | TODO: Add examples with stat/df | | TODO: Add examples with stat/df |
| | | | |
| − | [[Category: Troubleshooting]]
| + | TODO: Change to math: <math> avail_{ext2} <= free_{ext2} (1) </math> an alike |
| − | [[Category: Resource management]]
| |
| − | [[Category: Disk quota]]
| |
