From 10e3226ba13d184fc3165138c619eb7f2d52cdd2 Mon Sep 17 00:00:00 2001 From: Tomas Vondra Date: Fri, 5 Apr 2024 16:28:21 +0200 Subject: [PATCH] Align blocks in incremental backups to BLCKSZ Align blocks stored in incremental files to BLCKSZ, so that the incremental backups work well with CoW filesystems. The header of the incremental file is padded with \0 to a multiple of BLCKSZ, so that the block data (also BLCKSZ) is aligned to BLCKSZ. The padding is added only to files containing block data, so files with just the header remain small. This adds a bit of extra space, but as the number of blocks increases the overhead gets negligible very quickly. And as the padding is \0 bytes, it does compress extremely well. The alignment is important for CoW filesystems that usually require the blocks to be aligned to filesystem page size for features like block sharing, deduplication etc. to work well. With the variable sized header the blocks in the increments were not aligned at all, negating the benefits of the CoW filesystems. This matters even for non-CoW filesystems, for example when placed on a RAID array. If the block is not aligned, it may easily span multiple devices, causing read and write amplification. It might be better to align the blocks to the filesystem page, not BLCKSZ, but we have no good way to determine that. Even if we determine the page size at the time of taking the backup, the backup may move. For now the BLCKSZ seems sufficient - the filesystem page is usually 4K, so the default BLCKSZ (8K by default) is aligned to that. Author: Tomas Vondra Reviewed-by: Robert Haas, Jakub Wartak Discussion: https://postgr.es/m/3024283a-7491-4240-80d0-421575f6bb23%40enterprisedb.com --- src/backend/backup/basebackup.c | 26 ++++++++++++++ src/backend/backup/basebackup_incremental.c | 39 ++++++++++++++++++--- src/bin/pg_combinebackup/reconstruct.c | 8 +++++ src/include/backup/basebackup_incremental.h | 1 + 4 files changed, 70 insertions(+), 4 deletions(-) diff --git a/src/backend/backup/basebackup.c b/src/backend/backup/basebackup.c index 5fea86ad0f..b956df072d 100644 --- a/src/backend/backup/basebackup.c +++ b/src/backend/backup/basebackup.c @@ -1623,6 +1623,8 @@ sendFile(bbsink *sink, const char *readfilename, const char *tarfilename, { unsigned magic = INCREMENTAL_MAGIC; size_t header_bytes_done = 0; + char padding[BLCKSZ]; + size_t paddinglen; /* Emit header data. */ push_to_sink(sink, &checksum_ctx, &header_bytes_done, @@ -1635,6 +1637,23 @@ sendFile(bbsink *sink, const char *readfilename, const char *tarfilename, incremental_blocks, sizeof(BlockNumber) * num_incremental_blocks); + /* + * Add padding to align header to a multiple of BLCKSZ, but only if + * the incremental file has some blocks. If there are no blocks we + * don't want make the file unnecessarily large, as that might make + * some filesystem optimizations impossible. + */ + if (num_incremental_blocks > 0) + { + paddinglen = (BLCKSZ - (header_bytes_done % BLCKSZ)); + + memset(padding, 0, paddinglen); + bytes_done += paddinglen; + + push_to_sink(sink, &checksum_ctx, &header_bytes_done, + padding, paddinglen); + } + /* Flush out any data still in the buffer so it's again empty. */ if (header_bytes_done > 0) { @@ -1748,6 +1767,13 @@ sendFile(bbsink *sink, const char *readfilename, const char *tarfilename, blkno += cnt / BLCKSZ; bytes_done += cnt; + /* + * Make sure incremental files with block data are properly aligned + * (header is a multiple of BLCKSZ, blocks are BLCKSZ too). + */ + Assert(!((incremental_blocks != NULL && num_incremental_blocks > 0) && + (bytes_done % BLCKSZ != 0))); + /* Archive the data we just read. */ bbsink_archive_contents(sink, cnt); diff --git a/src/backend/backup/basebackup_incremental.c b/src/backend/backup/basebackup_incremental.c index 2970dfe603..131598bade 100644 --- a/src/backend/backup/basebackup_incremental.c +++ b/src/backend/backup/basebackup_incremental.c @@ -928,6 +928,36 @@ GetFileBackupMethod(IncrementalBackupInfo *ib, const char *path, return BACK_UP_FILE_INCREMENTALLY; } +/* + * Compute the size for a header of an incremental file containing a given + * number of blocks. The header is rounded to a multiple of BLCKSZ, but + * only if the file will store some block data. + */ +extern size_t +GetIncrementalHeaderSize(unsigned num_blocks_required) +{ + size_t result; + + /* Make sure we're not going to overflow. */ + Assert(num_blocks_required <= RELSEG_SIZE); + + /* + * Three four byte quantities (magic number, truncation block length, + * block count) followed by block numbers. + */ + result = 3 * sizeof(uint32) + (sizeof(BlockNumber) * num_blocks_required); + + /* + * Round the header size to a multiple of BLCKSZ - when not a multiple of + * BLCKSZ, add the missing fraction of a block. But do this only if the + * file will store data for some blocks, otherwise keep it small. + */ + if ((num_blocks_required > 0) && (result % BLCKSZ != 0)) + result += BLCKSZ - (result % BLCKSZ); + + return result; +} + /* * Compute the size for an incremental file containing a given number of blocks. */ @@ -940,11 +970,12 @@ GetIncrementalFileSize(unsigned num_blocks_required) Assert(num_blocks_required <= RELSEG_SIZE); /* - * Three four byte quantities (magic number, truncation block length, - * block count) followed by block numbers followed by block contents. + * Header with three four byte quantities (magic number, truncation block + * length, block count) followed by block numbers, rounded to a multiple + * of BLCKSZ (for files with block data), followed by block contents. */ - result = 3 * sizeof(uint32); - result += (BLCKSZ + sizeof(BlockNumber)) * num_blocks_required; + result = GetIncrementalHeaderSize(num_blocks_required); + result += BLCKSZ * num_blocks_required; return result; } diff --git a/src/bin/pg_combinebackup/reconstruct.c b/src/bin/pg_combinebackup/reconstruct.c index 41f06bb26b..33c6da02a8 100644 --- a/src/bin/pg_combinebackup/reconstruct.c +++ b/src/bin/pg_combinebackup/reconstruct.c @@ -472,6 +472,14 @@ make_incremental_rfile(char *filename) sizeof(rf->truncation_block_length) + sizeof(BlockNumber) * rf->num_blocks; + /* + * Round header length to a multiple of BLCKSZ, so that blocks contents + * are properly aligned. Only do this when the file actually has data for + * some blocks. + */ + if ((rf->num_blocks > 0) && ((rf->header_length % BLCKSZ) != 0)) + rf->header_length += (BLCKSZ - (rf->header_length % BLCKSZ)); + return rf; } diff --git a/src/include/backup/basebackup_incremental.h b/src/include/backup/basebackup_incremental.h index ae5feb4b32..6ba9c9035e 100644 --- a/src/include/backup/basebackup_incremental.h +++ b/src/include/backup/basebackup_incremental.h @@ -51,5 +51,6 @@ extern FileBackupMethod GetFileBackupMethod(IncrementalBackupInfo *ib, BlockNumber *relative_block_numbers, unsigned *truncation_block_length); extern size_t GetIncrementalFileSize(unsigned num_blocks_required); +extern size_t GetIncrementalHeaderSize(unsigned num_blocks_required); #endif