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On 08/13/2009 06:23 AM, Simo Sorce wrote:
during this month I have been slowly working on a set of patches to move
from storing information in 2 different formats (legacy and
member/memberOf based) to just one format (member/memberOf based).
While doing this I had to address some problems that come up when you
want to store a group and its members have not been stored yet, and
cases like this.
All the while I have been testing doing enumerations against a server
that has more than 3k users and 3k groups.
This is a medium sized database, and yet getting groups from scratch
(startup after deleting the .ldb database) could take up to a minute;
granted the operation is quite a bit faster if the database just needs
updating and not creation from scratch, but I still think it's too much.
I've been thinking hard about how to address this problem and solve the
few hacks we have in the code when it comes to enumeration caching and
retrieval. We always said that enumerations are evil (and they are
indeed) and in fact we even have options that disable enumerations by
default. Yet I think this is not necessarily the right way to go.
I think we have 2 major problems in our current architecture when it
comes to enumerations.
1) we try to hit the wire when an enumeration request comes in from a
process and a (small) timeout for the previous enumeration has been
2) We run the enumeration in a single transaction (and yes I have
recently introduced this), which means any other operation is blocked
until the enumeration is finished.
The problem I actually see is that user space apps may have to wait just
too much, and this *will* turn out to be a problem. Even if we give the
option to turn off enumeration I think that for apps that needs it the
penalty has become simply too big. Also I think the way we have to
perform updates using this model is largely inefficient, as we basically
perform a full new search potentially every few minutes.
One potential idea would be to have the SSSD automatically start an
enumeration at startup time if the cache is stale. Then, instead of
blocking updates waiting for subsequent enumerations, we could just go
immediately to the cache until the enumeration was complete.
After some hard thinking I wrote down a few points I'd like the
opinion on. If people agree I will start acting on them.
* stop performing enumerations on demand, and perform them in background
if enumerations are activated (change the enumeration parameter from a
bitfield to a true/flase boolean)
* perform a full user+group enumeration at startup (possibly using a
paged or vlv search)
* when possible request the modifyTimestamp attribute and save the
highest modifyTimestamp into the domain entry as originalMaxTimestamp
* on a tunable interval run a task that refreshes all users and groups
in the background using a search filter that includes
* still do a full refresh every X minutes/hours
* disable using a single huge transaction for enumerations (we might be
ok doing a transaction for each page search if pages are small,
otherwise just revert to the previous behavior of having a transaction
per stored object)
* Every time we update an entry we store the originalModifyTimestamp on
it as a copy of the remote modifyTimestamp, this allows us to know if we
actually need to touch the cached entry at all upon refresh (like when a
getpwnam() is called, speeding up operations for entries that need no
refresh (we will avoid data transformation and a writing to ldb).
* Every time we run the general refresh task or we save a changed entry
we store a LastUpdatedTimestamp
* When the refresh task is completed successfully we run another cleanup
task that searches our LDB for any entry that has a too old
LastUpdatedTimestamp. If any is found, we double check against the
remote server if the entry still exists (and update it if it does), and
otherwise we delete it.
I do like the idea of updating only the entries on the remote server
that have been updated since the previous enumeration.
NOTE: this means that until the first background enumeration is
complete, a getent passwd or a getent group call may return incomplete
results. I think this is acceptable as it will really happen only at
startup, when the daemon caches are empty.
I disagree. If we're going to have a startup enumeration, then we should
simply not enable handling NSS requests until that first enumeration is
complete. Incomplete results can be worse than no results. I assume NSS
has a return code for temporary failure?
NOTE2: Off course the scheduled refreshes and cleanup tasks are
rescheduled if we are offline or if a fatal error occurs during the
NOTE3: I am proposing to change only the way enumerations are
single user or group lookups will remain unchanged for now and will be
dealt with later if needed.
Please provide comments or questions if you think there is anything not
clear with the proposed items or if you think I forgot to take some
important aspect in account.
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