# Does Docker use UnionFS, and does it cause "fallocate:: Operation not supported"?

**URL:** <https://forums.percona.com/t/does-docker-use-unionfs-and-does-it-cause-fallocate-operation-not-supported/8161>\
**Category:** Percona Operator for MongoDB\
**Created:** [October 20, 2020, 6:51pm UTC](https://forums.percona.com/t/does-docker-use-unionfs-and-does-it-cause-fallocate-operation-not-supported/8161 "2020-10-20T18:51:40Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Akira\_Kurogane](https://sea1.discourse-cdn.com/flex019/user_avatar/forums.percona.com/akira_kurogane/32/1280_2.png) [@Akira\_Kurogane](https://forums.percona.com/u/Akira_Kurogane)\
**Post date:** [October 20, 2020, 6:51pm UTC](https://forums.percona.com/t/does-docker-use-unionfs-and-does-it-cause-fallocate-operation-not-supported/8161/1 "2020-10-20T18:51:40Z")

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In a question that was originally opened under the forum for PSMDB ([link](https://forums.percona.com/discussion/56154/wiredtiger-error-error-95-fallocate-operation-not-supported)) the following error was observed when using PSMDB 4.4 in the operator.

```auto
psmdb_1 | {"t":{"$date":"2020-09-01T19:50:00.963+00:00"},"s":"E", "c":"STORAGE", "id":22435, "ctx":"thread1","msg":"WiredTiger error","attr":{"error":95,"message":"[1598989800:963675][1:0x7f882be42700], log-server: __posix_std_fallocate, 58: /data/db/journal/WiredTigerTmplog.0000000003: fallocate:: Operation not supported"}}
psmdb_1 | {"t":{"$date":"2020-09-01T19:50:00.963+00:00"},"s":"E", "c":"STORAGE", "id":22435, "ctx":"thread1","msg":"WiredTiger error","attr":{"error":95,"message":"[1598989800:963758][1:0x7f882be42700], log-server: __posix_sys_fallocate, 75: /data/db/journal/WiredTigerTmplog.0000000003: fallocate:: Operation not supported"}}

```

As the error message shows the unix syscall fallocate() simply failed. MongoDB might retry a few times, but if can’t get file space allocated it has to give up, and error-exits.

The following Stackoverflow QA suggests the syscall failure is a limitation of docker: [https://stackoverflow.com/questions/31155591/fallocate-failing-inside-my-docker-container](https://stackoverflow.com/questions/31155591/fallocate-failing-inside-my-docker-container) .

Is this a true problem, and does it affect PSMDB operator now?

If this isn’t the cause then I guess fallocate() is failing because there truly is no filesystem space to allocate. How would we debug that in kubernetes?

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**Author:** ![Sergey\_Pronin](https://sea1.discourse-cdn.com/flex019/user_avatar/forums.percona.com/sergey_pronin/32/14887_2.png) [@Sergey\_Pronin](https://forums.percona.com/u/Sergey_Pronin)\
**Post date:** [November 9, 2020, 3:08pm UTC](https://forums.percona.com/t/does-docker-use-unionfs-and-does-it-cause-fallocate-operation-not-supported/8161/2 "2020-11-09T15:08:52Z")

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Hello @Akira_Kurogane ,

fallocate is supported by most of the filesystems. For MongoDB case specifically it depends where you store the data:

For example if you run MongoDB in docker without specifying any storage dirs - data is stored on default docker storage driver. In ubuntu it is overlay2. I tested fallocate - worked fine for me.

I replied in the original thread as well - would be great to see docker info first and understand where data is stored, what is the filesystem used.

For PSMDB Operator - we should be fine. It still depends where data is stored. But for most of the cases (AWS or GCP) Persistent Volume Claims use battle-tested filesystems like ext4 or xfs. They do support fallocate.

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<div class="post-metadata">

**Author:** ![Akira\_Kurogane](https://sea1.discourse-cdn.com/flex019/user_avatar/forums.percona.com/akira_kurogane/32/1280_2.png) [@Akira\_Kurogane](https://forums.percona.com/u/Akira_Kurogane)\
**Post date:** [November 10, 2020, 12:20am UTC](https://forums.percona.com/t/does-docker-use-unionfs-and-does-it-cause-fallocate-operation-not-supported/8161/3 "2020-11-10T00:20:52Z")

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Hi Sergey. Thanks for trying out the kubernetes environment part, that’s what I don’t know.

\> I tested fallocate [in an overlay2 filesystem-using case] - worked fine for me.

Thanks. So I guess we should rule out that it was just overlay2 fs alone. At most, it might be overlay2 filesystem in combination with transient resource failure such as a lack of memory.

Or … maybe it was overlay 1-not-2. I’ll ask the community user Stef for the docker info as well.