Simo Sorce wrote: > Hello all, > 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 > expired. >
May be then we should as I quick fix have a separate timeout for the enumerations? > 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. > Can we create a special back end for enumerations and separate it from individual operations? > 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. Agree. > 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. > > Agree, though I think that we can separate these enhancements and do them as a separate effort (may be later, after F12 if we do not have time). > After some hard thinking I wrote down a few points I'd like the list > 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) > Is a separate back end may be? > * perform a full user+group enumeration at startup (possibly using a > paged or vlv search) > Yes, I agree, but there is a concern (see below). > * 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 > &(modifyTimestamp>$originalMaxtimestamp) > Okey... Steven explain it in more details since i was concerned about the time stamp being also updated on individual refreshes but he said that this time stamp will be touched only by enumerations. Hm I see how that would work. > * still do a full refresh every X minutes/hours > Configurable. Sure. > * 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) > Can you do page - individual request - page -individual request? If yes then it makes sense to have transaction per page. If you have to do pages one after another and can't do other requests in the middle I do not see how changing transaction scope would help. > * 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). > Is this only for enumerations or on individual updates too? How it is related to the currently designed and being implemented cache logic? Can you please explain how this would affect the cache logic? > * 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. > > Makes sense. I actually makes me think that with out of band periodic enumerations and cleanups it becomes more and more appealing to have it in a separate back end. > 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. > > Here is my concern I mentioned above: How many services and daemons at startup rely on the enumeration? Do we know? Is there any way to know? Should we ask the communities about those to make sure we meet the expectations? What about things line network manager, HAL, System bus, auditd, GDE and many other processes that start at boot? If we block them it might be a show stopper if we provide partial data it might be Ok might be not. I guess we need more input on the matter. Do you agree? > NOTE2: Off course the scheduled refreshes and cleanup tasks are always > rescheduled if we are offline or if a fatal error occurs during the > task. > > IMO this is yet another reason to have a separate back end. > NOTE3: I am proposing to change only the way enumerations are handled, > single user or group lookups will remain unchanged for now and will be > dealt with later if needed. > > > Sure. I think the only impact is that the entry might be refreshed with the enumeration and we need to factor a new time stamp in the cache refresh logic. Other than that I do not see any impact. > 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. > > Simo. > > _______________________________________________ > sssd-devel mailing list > sssd-devel@lists.fedorahosted.org > https://fedorahosted.org/mailman/listinfo/sssd-devel > -- Thank you, Dmitri Pal Engineering Manager IPA project, Red Hat Inc. ------------------------------- Looking to carve out IT costs? www.redhat.com/carveoutcosts/ _______________________________________________ sssd-devel mailing list sssd-devel@lists.fedorahosted.org https://fedorahosted.org/mailman/listinfo/sssd-devel
