Head-truncate time series files only if a certain percentage of size decrease is accomplished

[beorn7]

Servers with a very long retention period have series files with a lot of chunks. Even if only a single chunk at the beginning of the file is removed in series maintenance, the whole file is rewritten. This causes a lot of I/O for very little effect. On spinning disk, the problem doesn't show up that much because series maintenance takes long, and therefore larger changes are batched up implicitly. On SSDs, though, series maintenance is so fast and thus performed so frequently that many of those micro truncations can happen.

Some filesystems support "punching holes", which can be used to head-truncate without rewriting the file. However, that might cause more load for the filesystem, and it's not a general solution as not all filesystems support it.

An easy solution is to only rewrite the series file if a certain percentage of the total size is head-truncated. With e.g. 10%, the overhead in wasted disk space is limited but the I/O load will be reduced quite a bit.

[lock]

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