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Overview
Comment: | Fix typos and make improvements to the 35% faster document. |
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4764f16dc04e1db9cf675fb8b841e300 |
User & Date: | drh 2017-05-04 20:14:45.970 |
Context
2017-05-04
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20:23 | More tweaks to the 35% faster document. (check-in: 3563b95114 user: drh tags: trunk) | |
20:14 | Fix typos and make improvements to the 35% faster document. (check-in: 4764f16dc0 user: drh tags: trunk) | |
19:44 | Add the "35% Faster Than The Filesystem" document. (check-in: fd906f77ce user: drh tags: trunk) | |
Changes
Changes to pages/fasterthanfs.in.
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20 21 22 23 24 25 26 | when reading the blobs from individual files. In other words, the overhead of calling open() and close() is greater than the overhead of using the database. The size reduction arises from the fact that individual files are padded out to the next multiple of the filesystem block size, whereas the blobs are packed more tightly into an SQLite database. | | > > > > > > > | > > > > > > > > | 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 | when reading the blobs from individual files. In other words, the overhead of calling open() and close() is greater than the overhead of using the database. The size reduction arises from the fact that individual files are padded out to the next multiple of the filesystem block size, whereas the blobs are packed more tightly into an SQLite database. <h1>How Performance Was Measured</h1> <p>The performance comparison is accomplished using the [https://www.sqlite.org/src/file/test/kvtest.c|kvtest.c] program found in the SQLite source tree. To compile the test program, first gather the kvtest.c source file into a directory with the [amalgamation|SQLite amalgamation] source files "sqlite3.c" and "sqlite3.h". Then on unix, run a command like the following: <codeblock> gcc -Os -I. -DSQLITE_DIRECT_OVERFLOW_READ kvtest.c sqlite3.c \ -o kvtest -ldl -lpthread </codeblock> <p>Or on Windows with MSVC: <codeblock> cl -I. -DSQLITE_DIRECT_OVERFLOW_READ kvtest.c sqlite3.c </codeblock> <p> Use the resulting "kvtest" program to generate a test database with 100,000 random blobs, each 10,000 bytes in size using a command like this: <codeblock> ./kvtest init test1.db --count 100k --size 10k </codeblock> <p> Next, make copies of all the blobs into individual files in a directory using commands like this: <codeblock> mkdir test1.dir ./kvtest export test1.db test1.dir </codeblock> <p> At this point, you can measure the amount of disk space used by the test1.db database and the space used by the test1.dir directory and all of its content. On a standard Ubuntu Linux desktop, the database file will be 1,024,512,000 bytes in size and the test1.dir directory will use 1,228,800,000 bytes of space (according to "du -k"), about 20% more than the database. <p> Measure the performance for reading blobs from the database and from individual files using these commands: <codeblock> ./kvtest run test1.db --count 100k --blob-api ./kvtest run test1.dir --count 100k |
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89 90 91 92 93 94 95 | will be amortized over more bytes transferred using read(). The break-even point, the point where it becomes faster to read directly from the filesystem, will vary from one system to another. <p>The --blob-api option causes database reads to occur using the [sqlite3_blob_open()], [sqlite3_blob_reopen()], and [sqlite3_blob_read()] interfaces instead of using SQL statements. Without the --blob-api | | | | | | | 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 | will be amortized over more bytes transferred using read(). The break-even point, the point where it becomes faster to read directly from the filesystem, will vary from one system to another. <p>The --blob-api option causes database reads to occur using the [sqlite3_blob_open()], [sqlite3_blob_reopen()], and [sqlite3_blob_read()] interfaces instead of using SQL statements. Without the --blob-api option, a separate SQL statement is run to read each blob and the performance of reading from the database is approximately the same as the performance from reading directly from files. This is still a significant finding, since few people would expect a [full-featured SQL] database to run as fast as direct file reads, and yet SQLite does. <p>If the --random option is added to the "run" command, that causes the blobs to be read in a random order. This causes a noticable decrease in the performance of database reads, since it forces more movement of B-Tree cursors. When --random is used and --blob-api is omitted, reading directly from files on disk is generally a little faster, but reads from the database are still competitive. <h1>Other Considerations</h1> <p>Some other SQL database engines advise developers to store blobs in separate files and then store the filename in the database. In that case, where the database must first be consulted to find the filename before opening and reading the file, simply storing the entire blob in the database is gives much faster read performance with SQLite. See the [Internal Versus External BLOBs] article for more information. <p>This report only looks at the performance of reads, not writes. |
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