SLO: Concurrently HEAD segments

Before creating a static large object, we must verify that all of the
referenced segments exist. Previously, this was done sequentially; due
to latency between proxy and object nodes, clients must be careful to
either keep their segment count low or use very long (minute+) timeouts.
We mitigate this somewhat by enforcing a hard limit on segment count,
but even then, HEADing a thousand segments (the default limit) with an
average latency of (say) 100ms will require more than a minute and a
half.

Further, the nested-SLO approach requires multiple requests from the
client -- as a result, Swift3 is in the position of enforcing a lower
limit than S3's 10,000 (which will break some clients) or requiring that
clients have timeouts on the order of 15-20 minutes (!).

Now, we'll perform the segment HEADs in parallel, with a concurrency
factor set by the operator. This is very similar to (and builds upon)
the parallel-bulk-delete work. By default, two HEAD requests will be
allowed at a time.

As a side-effect, we'll also only ever HEAD a path once per manifest.
Previously, if a manifest alternated between two paths repeatedly (for
instance, because the user wanted to splice together various ranges from
two sub-SLOs), then each entry in the manifest would trigger a fresh
HEAD request.

Upgrade Consideration
=====================
If operators would like to preserve the prior (single-threaded) SLO
creation behavior, they must add the following line to their
[filter:slo] proxy config section:

   concurrency = 1

This may be done prior to upgrading Swift.

UpgradeImpact
Closes-Bug: #1637133
Related-Change: I128374d74a4cef7a479b221fd15eec785cc4694a
Change-Id: I567949567ecdbd94fa06d1dd5d3cdab0d97207b6

0 files changed, 0 insertions, 0 deletions