I tested four NVMe SSDs from four vendors – half lose FLUSH'd data on power loss

Re:I tested four NVMe SSDs from four vendors – half lose FLU... by Filligree on Wednesday November 22, 2023 @12:41AM (#38375124)

The file isn't stored contiguously on disk, so that would depend on the implementation of the filesystem. Perhaps the size of the file can be changed, without extents necessarily being allocated to cover the new size?

I seem to vaguely recall an issue like that, for ext4 in particular. Of course it's possible in general for any filesystem that supports holes, but I don't think we can necessarily assume that the data is always written, and all the pointers to it also written, before the file-size gets updated.

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Re:I tested four NVMe SSDs from four vendors – half lose FLU... by lxgr on Wednesday November 22, 2023 @12:49AM (#38375156)

At least for ext4 and actually written data (i.e. not ftruncate’d files), I believe zeroes should really not occur.

Both extents and the file size are metadata as far as I understand, which would be atomically updated through the journal.

Data can be written before metadata (in data=ordered mode):

> All data are forced directly out to the main file system prior to its metadata being committed to the journal.

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Re:I tested four NVMe SSDs from four vendors – half lose FLU... by Filligree on Wednesday November 22, 2023 @01:02AM (#38375245)

Is data=ordered the default?

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Re:I tested four NVMe SSDs from four vendors – half lose FLU... by lxgr on Wednesday November 22, 2023 @09:13AM (#38379409)

As far as I can tell it's always been on both ext3 and ext4, yes.

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