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bup-margin(1) [debian man page]

bup-margin(1)						      General Commands Manual						     bup-margin(1)

NAME
bup-margin - figure out your deduplication safety margin SYNOPSIS
bup margin [options...] DESCRIPTION
bup margin iterates through all objects in your bup repository, calculating the largest number of prefix bits shared between any two entries. This number, n, identifies the longest subset of SHA-1 you could use and still encounter a collision between your object ids. For example, one system that was tested had a collection of 11 million objects (70 GB), and bup margin returned 45. That means a 46-bit hash would be sufficient to avoid all collisions among that set of objects; each object in that repository could be uniquely identified by its first 46 bits. The number of bits needed seems to increase by about 1 or 2 for every doubling of the number of objects. Since SHA-1 hashes have 160 bits, that leaves 115 bits of margin. Of course, because SHA-1 hashes are essentially random, it's theoretically possible to use many more bits with far fewer objects. If you're paranoid about the possibility of SHA-1 collisions, you can monitor your repository by running bup margin occasionally to see if you're getting dangerously close to 160 bits. OPTIONS
--predict Guess the offset into each index file where a particular object will appear, and report the maximum deviation of the correct answer from the guess. This is potentially useful for tuning an interpolation search algorithm. --ignore-midx don't use .midx files, use only .idx files. This is only really useful when used with --predict. EXAMPLE
$ bup margin Reading indexes: 100.00% (1612581/1612581), done. 40 40 matching prefix bits 1.94 bits per doubling 120 bits (61.86 doublings) remaining 4.19338e+18 times larger is possible Everyone on earth could have 625878182 data sets like yours, all in one repository, and we would expect 1 object collision. $ bup margin --predict PackIdxList: using 1 index. Reading indexes: 100.00% (1612581/1612581), done. 915 of 1612581 (0.057%) SEE ALSO
bup-midx(1), bup-save(1) BUP
Part of the bup(1) suite. AUTHORS
Avery Pennarun <apenwarr@gmail.com>. Bup unknown- bup-margin(1)

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bup-memtest(1)						      General Commands Manual						    bup-memtest(1)

NAME
bup-memtest - test bup memory usage statistics SYNOPSIS
bup memtest [options...] DESCRIPTION
bup memtest opens the list of pack indexes in your bup repository, then searches the list for a series of nonexistent objects, printing memory usage statistics after each cycle. Because of the way Unix systems work, the output will usually show a large (and unchanging) value in the VmSize column, because mapping the index files in the first place takes a certain amount of virtual address space. However, this virtual memory usage is entirely virtual; it doesn't take any of your RAM. Over time, bup uses parts of the indexes, which need to be loaded from disk, and this is what causes an increase in the VmRSS column. OPTIONS
-n, --number=number set the number of objects to search for during each cycle (ie. before printing a line of output) -c, --cycles=cycles set the number of cycles (ie. the number of lines of output after the first). The first line of output is always 0 (ie. the base- line before searching for any objects). --ignore-midx ignore any .midx files created by bup midx. This allows you to compare memory performance with and without using midx. --existing search for existing objects instead of searching for random nonexistent ones. This can greatly affect memory usage and performance. Note that most of the time, bup save spends most of its time searching for nonexistent objects, since existing ones are probably in unmodified files that we won't be trying to back up anyway. So the default behaviour reflects real bup performance more accurately. But you might want this option anyway just to make sure you haven't made searching for existing objects much worse than before. EXAMPLE
$ bup memtest -n300 -c5 PackIdxList: using 1 index. VmSize VmRSS VmData VmStk 0 20824 kB 4528 kB 1980 kB 84 kB 300 20828 kB 5828 kB 1984 kB 84 kB 600 20828 kB 6844 kB 1984 kB 84 kB 900 20828 kB 7836 kB 1984 kB 84 kB 1200 20828 kB 8736 kB 1984 kB 84 kB 1500 20828 kB 9452 kB 1984 kB 84 kB $ bup memtest -n300 -c5 --ignore-midx PackIdxList: using 361 indexes. VmSize VmRSS VmData VmStk 0 27444 kB 6552 kB 2516 kB 84 kB 300 27448 kB 15832 kB 2520 kB 84 kB 600 27448 kB 17220 kB 2520 kB 84 kB 900 27448 kB 18012 kB 2520 kB 84 kB 1200 27448 kB 18388 kB 2520 kB 84 kB 1500 27448 kB 18556 kB 2520 kB 84 kB DISCUSSION
When optimizing bup indexing, the first goal is to keep the VmRSS reasonably low. However, it might eventually be necessary to swap in all the indexes, simply because you're searching for a lot of objects, and this will cause your RSS to grow as large as VmSize eventually. The key word here is eventually. As long as VmRSS grows reasonably slowly, the amount of disk activity caused by accessing pack indexes is reasonably small. If it grows quickly, bup will probably spend most of its time swapping index data from disk instead of actually running your backup, so backups will run very slowly. The purpose of bup memtest is to give you an idea of how fast your memory usage is growing, and to help in optimizing bup for better memory use. If you have memory problems you might be asked to send the output of bup memtest to help diagnose the problems. Tip: try using bup midx -a or bup midx -f to see if it helps reduce your memory usage. Trivia: index memory usage in bup (or git) is only really a problem when adding a large number of previously unseen objects. This is because for each object, we need to absolutely confirm that it isn't already in the database, which requires us to search through all the existing pack indexes to ensure that none of them contain the object in question. In the more obvious case of searching for objects that do exist, the objects being searched for are typically related in some way, which means they probably all exist in a small number of pack- files, so memory usage will be constrained to just those packfile indexes. Since git users typically don't add a lot of files in a single run, git doesn't really need a program like bup midx. bup, on the other hand, spends most of its time backing up files it hasn't seen before, so its memory usage patterns are different. SEE ALSO
bup-midx(1) BUP
Part of the bup(1) suite. AUTHORS
Avery Pennarun <apenwarr@gmail.com>. Bup unknown- bup-memtest(1)
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