Re: [gentoo-dev] New distfile mirror layout

[Richard Yao]

> On Oct 18, 2019, at 4:49 PM, Michał Górny <mgorny@g.o> wrote:
>
> On Fri, 2019-10-18 at 15:53 -0400, Richard Yao wrote:
>>>>>> On Oct 18, 2019, at 9:42 AM, Michał Górny <mgorny@g.o> wrote:
>>>>> Hi, everybody.
>>>>> It is my pleasure to announce that yesterday (EU) evening we've switched
>>>>> to a new distfile mirror layout.  Users will be switching to the new
>>>>> layout either as they upgrade Portage to 2.3.77 or -- if they upgraded
>>>>> already -- as their caches expire (24hrs).
>>>>> The new layout is mostly a bow towards mirror admins, for some of whom
>>>>> having a 60000+ files in a single directory have been a problem.
>>>>> However, I suppose some of you also found e.g. the directory index
>>>>> hardly usable due to its size.
>> This sounds like a filesystem issue. Do we know which filesystems are suffering?
>> ZFS should be fine. I believe ext2/ext3 have problems with this many files. ext4 is probably okay, but don’t quote me on that.
>
> Ext2, VFAT and NTFS were mentioned on the bug [1], though I suppose this
> may apply only to older ntfs versions.  NFS has been mentioned too.

ext2 and vfat are not surprises to me (outside of the idea that anyone would use them for a mirror). NTFS and NFS are though.
>
> However, just because modern filesystems can handle them efficiently, it
> doesn't mean having directories that huge comes with zero cost.
While I am okay with the change, what do you mean when you say that having huge directories does not come with zero cost?

Filesystems with O(1) directory lookups like ZFS would probably be hurt by this, but the impact should be negligible. Filesystems with O(log n) directory lookups would see faster directory lookups.

Outside of directory lookups, this could speed up up searches and sort operations when listing everything with just about any filesystem benefiting from the improvement.

Listing directories on such filesystems should not benefit from this unless you are using ls where the default behavior is to sort the directory contents (which is where the improvement when sorting comes into play). The need to sort the directory contents by default keeps ls from displaying anything until it has scanned the entire directory. The asymptotic complexity of a fast comparison based sort improves in this situation from O(nlogn) to O(nlog(n/b)) provided that you sort each subdirectory independently. A further speed up could be obtained by doing multithreading to parallelize the sort operations.

Since I know someone will call me out on that comment, I will explain. Each bucket has roughly n/b items in it where n is the total number and b is the number of buckets. Sorting one bucket is O(n/b * log(n/b)). Loop to sort each of the b buckets. The buckets are pre-sorted by prefix, so the result is now sorted. You therefore get O(nlog(n/b)) time complexity out of an O(nlogn) comparison sort on this very special case where you call it multiple times on data that has been persorted by prefix into buckets.

Is there any other benefit to this or did I get everything?

By the way, it is offtopic for the thread, but it occurs to me that a hybrid of radix sort and A comparison based sort could give us a general sorting algorithm that is asymptotically faster than O(nlogn).
>
> [1] https://bugs.gentoo.org/534528
>
> --
> Best regards,
> Michał Górny