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README.md: information on metadata size

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Vincent Batts 2015-03-10 11:41:20 -04:00
parent 402c6217ac
commit 61b11c52f8

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@ -113,6 +113,56 @@ ca9e19966b892d9ad5960414abac01ef585a1e22 tar-split.tar
ca9e19966b892d9ad5960414abac01ef585a1e22 tar-split.tar.out ca9e19966b892d9ad5960414abac01ef585a1e22 tar-split.tar.out
``` ```
Stored Metadata
---------------
Since the raw bytes of the headers and padding are stored, you may be wondering
what the size implications are. The headers are at least 512 bytes per
file (sometimes more), at least 1024 null bytes on the end, and then various
padding. This makes for a constant linear growth in the stored metadata, with a
naive storage implementation.
Reusing our prior example's `tar-split.tar`, let's build the checksize.go example:
```
go build ./checksize.go
```
```
$ ./checksize ./tar-split.tar
inspecting "tar-split.tar" (size 210k)
-- number of files: 50
-- size of metadata uncompressed: 53k
-- size of gzip compressed metadata: 3k
```
So assuming you've managed the extraction of the archive yourself, for reuse of
the file payloads from a relative path, then the only additional storage
implications are as little as 3kb.
But let's look at a larger archive, with many files.
```
$ ls -sh ./d.tar
1.4G ./d.tar
$ ./checksize ~/d.tar
inspecting "/home/vbatts/d.tar" (size 1420749k)
-- number of files: 38718
-- size of metadata uncompressed: 43261k
-- size of gzip compressed metadata: 2251k
```
Here, an archive with 38,718 files has a compressed footprint of about 2mb.
Rolling the null bytes on the end of the archive, we will assume a
bytes-per-file rate for the storage implications.
| uncompressed | compressed |
| :----------: | :--------: |
| ~ 1kb per/file | 0.06kb per/file |
What's Next? What's Next?
------------ ------------