overlay: try harder with more layers #1375
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nalind
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This is making me consider if we'd just be better off recording layer IDs in "${layer}/lower" in "${layer}/diff" form and dispensing with the symbolic link farm in "l" entirely. We'd fork/exec for mounting a lot more often. |
drivers/overlay/mount.go
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| // paths, but we don't want to mess with other options. | ||
| var upper, work, lower, label, other string | ||
| for _, arg := range strings.Split(options.Label, ",") { | ||
| if strings.HasPrefix(arg, "upperdir=") { |
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Style only:
nv := strings.SplitN(arg, "=")
switch nv[0] {
case "upperdir":
upper = nv[1]
…
}(and eventually strings.Cut with Go 1.18.)
Separately: The caller has all of this data. It feels a bit tedious to format it into a single string, and then parse it out again — but changing that would be a rather larger rewrite.
drivers/overlay/mount.go
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| // Reconstruct the Label field. | ||
| options.Label = "" | ||
| for _, arg := range []string{upper, work, lower, label, other} { |
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[A] …
something like
for _, pair := range []struct{optionName, value string} {
{ "upperdir=", upper },
…
{ "", other },
}would allow most of this function to only deal with the individual values instead of the mount syntax.
(Ultimately, this “turn option values into a mount option syntax string” code might be useful to completely factor out…)
If we have enough layers that even using relative paths for symlinks to the diff directories won't make the list of lower directories and options fit in a single page, * open the various lowers for reading * use the descriptor names in the lowerdir= list This is a bit slower, so don't take these extra steps unless we know we have to. Signed-off-by: Nalin Dahyabhai <[email protected]>
nalind
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@mtrmac PTAL again. |
| // is true (512 is a buffer for label metadata). | ||
| // is true (512 is a buffer for label metadata, 128 is the | ||
| // number of lowers we want to be able to use without having | ||
| // to use bind mounts to get all the way to the kernel limit). | ||
| // ((idLength + len(linkDir) + 1) * maxDepth) <= (pageSize - 512) |
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Now the inequality does not hold. s/maxDepth/128/ or maybe … maxDirect = 128 is the … and * maxDirect?
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Opened #1392 to do the first one.
If we have enough layers that even using relative paths for symlinks to the diff directories won't make the list of lower directories and options fit in a single page,
create a temporary directorymount a tmpfs therebind mount from the diff directories to subdirectories of the temp directory with short namesmount using those bind-mounted directories as the lower directoriesdetach the temporary directoryremove the temporary directorylowerdir=listThis is
noticeablyslower, so don't take these extra steps unless we know we absolutely have to.Should help with cri-o/cri-o#6261.