On Mon, Apr 18, 2022 at 02:50:17PM +0530, Bharath Rupireddy wrote: > Thanks for sharing this. Will the write operations (in > dir_open_for_write) for PG_COMPRESSION_GZIP and PG_COMPRESSION_LZ4 > take longer compared to prepadding for non-compressed files?
The first write operations for gzip and lz4 consists in writing their respective headers in the resulting file, which should be a couple of dozen bytes, at most. So that's surely going to be cheaper than the pre-padding done for a full segment with the flush induced after writing WalSegSz bytes worth of zeros. > I would like to know if there's any problem with the proposed fix. There is nothing done for the case of compressed segments, meaning that you would see the same problem when being in the middle of writing a segment compressed with gzip or lz4 in the middle of writing it, and that's what you want to avoid here. So the important part is not the pre-padding, it is to make sure that there is enough space reserved for the handling of a full segment before beginning the work on it. -- Michael
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