5/14 thread: full fsck req'd after BC split

[Misa]

In context of the above-mentioned thread in the business recovery planning section, I ask this question again -- why is a full fsck required?

I use only EMC equipment, and based on experience there, I would have said he might need to "sync;sync" (or sync;sync;sync if you're really old) before splitting. With EMC, there is no need to unmount the original filesystems before initiating the split; I only stop the relevant apps that are writing.

I'm only assuming that the reference to "BC" means HP's XP. I'm trying to learn more about HP storage so I can evaluate HP vs EMC.

Can someone from an HP storage world address my question/point out my mis-assumptions? TIA.

--M????a

[Bill Hassell]

It has nothing to do with the hardware. fsck is required because the filesystem was not unmounted properly before the split. This is filesystem logic. The disks are just fine but the directory is partly in memory and aprtly on the disk. Stopping apps that are writing reduces the difficulties in cleaning up the filesystem, but there are dozens of ways that a filesystem can be 'busy' including the (poor but common) practice of cd'ing into every directory and not realizing that the directory is now open, just like a file. Split the disk mirror and you will have a dirty filesystem.

And most file writes do not stop when the app stops...there's all that data in the buffer cache that must be flushed out.

That's why umount takes a while before completing on a busy filesystem. So, no matter how many syncs are done, the filesystem was never un-mounted when it is split and there is no way for the opsystem to know if the entries are still valid without checking every entry. The simple concept of writing to a disk went away about a decade or two ago.

[Misa]

My question is not actually "why is any fsck required," but rather, "why did a full fsck seem to be required in this situation."

I do what I described in my previous post -- stop apps, sync, split -- and I never have to do a full fsck. I wanted to know if it were a requirement of XP or simply a result of the way this individual handled the systems. It sounds like the latter.

I do want to figure out whether XP storage will do what I need, and then, whether it's worthwhile to replace EMC with it.

[George Petrides_1]

OK, let's see how the filesystem info works. The information regarding files (inode table) is kept in memory and the table is flashed to the disk (by the sync command) automatically every 30 seconds. Now... if a server goes down for example, when it comes up there is a flag still up saying "the filesystem was not properly unmounted" and therefore corruption might have occured since information in memory was not flashed to the disk. An example would be a file that was deleted, the info got updated in the inode table in memory but was not flushed to the table on the disk. Therefore, fsck is ran to go through the actual filesystem contents and compare it to the inode table. In the example above of the file that was deleted, it will find an orphaned inode that is pointing to a non existent file and will of course clear the inode. So this is the theory, I hope EMC people are not using this to tell you their hardware is better since you will always have to do fsck unless you unmount the file system before the split and that will clear the 'possible corruption flag'. And as you can see from the explanation above, by manually typing sync it cannot guarantee file system integrity 100%.

[Misa]

I'm sorry if I'm not being clear.

My issue is why is a FULL fsck required. In my four years of experience with EMC hardware, using the procedure outlined above, I have never had to do a FULL one. fsck, yes, of course, but not FULL. Not -o full. And I certainly don't need to unmount things.

--Misa

[Martin Johnson]

You seem to me to be the type who would walk the highwire without a safety net, which is what you are doing with your data.

A full fsck validates the integrety of your data. Something you really want to do prior to backing it up. You are betting your business on the ability to successfully recover your data in the event of a disaster.

Not doing a umount before the split is an unnecessary risk.

IMHO
Marty

[Misa]

Well, I have to admit that being unfairly accused of something by someone who knows very little about me has certainly put a sour taste in my mouth wrt the forums. I'm a newcomer to the HP forums and this is not something that encourages me to come back.

I am legitimately trying to understand the reason everyone is saying this because it flies in the face of all my experience. I'm currently investigating the topic with some HP experts and if I can come up with a detailed technical basis for supporting either position, I'll try to post it.

--Misa

[Martin Johnson]

A person is judged by his or her actions. I feel you are taking unnecessary risks with your data.

Marty