The addition of range support for SLO segments (commit 25d5e68)
required the range size to be at least the SLO minimum segment size
(default 1 MiB). However, if you're doing something like assembling a
video of short clips out of a larger one, then you might not need a
full 1 MiB.
The reason for the 1 MiB restriction was to protect Swift from
resource overconsumption. It takes CPU, RAM, and internal bandwidth to
connect to an object server, so it's much cheaper to serve a 10 GiB
SLO if it has 10 MiB segments than if it has 10 B segments.
Instead of a strict limit, now we apply ratelimiting to small
segments. The threshold for "small" is configurable and defaults to 1
MiB. SLO segments may now be as small as 1 byte.
If a client makes SLOs as before, it'll still be able to download the
objects as fast as Swift can serve them. However, a SLO with a lot of
small ranges or segments will be slowed down to avoid resource
overconsumption. This is similar to how DLOs work, except that DLOs
ratelimit *every* segment, not just small ones.
UpgradeImpact
For operators: if your cluster has enabled ratelimiting for SLO, you
will want to set rate_limit_under_size to a large number prior to
upgrade. This will preserve your existing behavior of ratelimiting all
SLO segments. 5368709123 is a good value, as that's 1 greater than the
default max object size. Alternately, hold down the 9 key until you
get bored.
If your cluster has not enabled ratelimiting for SLO (the default), no
action is needed.
Change-Id: Id1ff7742308ed816038a5c44ec548afa26612b95
7f636a5572