Difference between lvm and mdadm replace
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Storage is always on fibrechannel, but fabric switches and disk arrays differ quite a lot depending on a client what deal they got, relationship with storage companies they have etc. Each environment spans 2 datacenters and those are interconnected by DWDM making the two DC networks both ip and fc appear as one.
Networks are of course split by vlans and fc zoning into smaller pieces. We use both hypervisor clustering and in-vm clusters. Why am describing all this? As you can see, such zoos are rather vibrant. I am grateful to have seen all this technology at work daily, for last almost 4 years my god time flies. I should not need to add, that in environments like these, there are often thousands of LVM volumes, and during you work you'll eventually touch all of them.
Oldest environment is fully LVM based and what can I say: it works, until it doesn't. The main problem I have with LVM is that if it does some of it's stupidities, you are on your own. It often happens when you least expect it or rather need it and in production environment not devevelopment, not testing or preproduction. Also commands are quite baroque, and kinda reversible but only to the point, when you start pumping data on the volume. Once that happens and you discovered a mistake only after, you should simply burn the volume and start a new.
It will be faster and probably more robust and you'll do less errors. Most jarring one was neophyte admin's extension of LVM stack by few hundredth Gigs of storage, which resulted in extended LV suddenly reporting -4trilions in size. The weird negative size of the volume made it impossible to run umount, fsck or any other fixing tools and introduced other problems.
Fortunately descending into directories still worked, so we rebuilt the whole VM again and used rsync to transfer mostly read only data. Data team then did analysis and they have not found any data loss - so it was probably just free space getting somehow mucked up. But the final result was that LVM caused such complication and locked volume in a way that not even basic data recovery tools were able to run. Also the original system was lost and had to be replaced and then dismantled. I and our architect, we did the analysis of commands issued, and it was done completely by the book, so I am not sure what happened there.
We also have miniscule amounts of LVM mirrors using cling extension to make LV sub-devices stick to proper datacenters on physical layer. This ensures that should cross DC link break that mirror would assemble at least on one side. All I will say is, that you don't want to be dealing with these setups in the middle of the night. We never had the balls to use LVM snapshots, despite them being supposedly fixed. There are many horror stories about them on the net and I am not willing to try them, especially since now we have tools to avoid these issues entirely.
Regarding a normal use, my major issue with LVM and overall state of linux filesystems, is their inability to check their own shit. I have not had time to dive deep into LVM mirroring, but I still have not found a person or explicit written confirmation, whether LVM mirror actually calculates checksums of the blocks and any checksum would do even crc So even if I run LVM mirror recalc, what is it actually doing?
Firstly, it is worth mentioning that when working with standard LVM volumes, you have to know a very large number of commands and utilities to be able to work correctly with the volumes, as there is no graphical shell. Secondly, when working with LVM, it is important to know which filesystems to use i.
Your choice of the filesystem has implications for adding new disks. In short, LVM is quite complex to maintain and requires the user to have a thorough knowledge of the operating system. All information in LVM is divided into extents these are blocks of information which are written to disk. The minimum size of an extent is 4 MB.
There are quite a few algorithms to write extents to disks. We will take the simplest one, called linear, as it is the easiest way to understand how LVM works. When using the linear algorithm, all extents are written to the disks in order. If there is not enough disk space to write the extents, they are written in order to the second one, and so on.
Read more: External Hard Drive not showing up in Windows That is, physically the file may be stored on several disks at the same time, but LVM subsystem remembers the order of the extents and thus knows in what order to read them to open the desired file. Schematically, it can be shown as follows: However, this is a disadvantage of LVM — since all extents are written sequentially — recording to the next SSD drive will start only after the first one is completely full. As you know, it is not recommended to fill SSD drives, because it severely degrades performance and reduces the performance of the garbage collector in other words, it reduces the speed and reduces the lifetime of the drive.
And here many will say that you can use striping method some analogue of RAID 0 in mdadm. However, there are also pitfalls here… The thing is that to expand our volume we will have to use a multiple of the number of disks already in use.
That is, if we used 3 disks, then we can add 3, 6, 9, etc. But the most important thing is that when adding a new group of disks, the striping will not be done between all disks at the same time, but within each disk group.
It turns out that recording to a new group of disks will begin only when the first one is full. Hence the conclusion that the first disk group will work under stress. Mdadm can solve this problem, since its new versions allow adding new disks to the RAID 0 array without any problems, after which writing to them will stripe with the previously installed disks, reducing the load and wear level for SSD drives.
For this reason we recommend that you use mdadm to create a software RAID, since it will be both faster and more reliable. There is an answer to this question and it is described in detail in the next paragraph of this article. Is it possible to use mdadm and LVM at the same time? We have already said that the best way to create RAID arrays is with the mdadm utility. That is, first you create a RAID of the required type e. It will provide reliability and speed, plus you can partition the disk space however you want.
Moreover, if you ever need to add a new disk to the system, you can do it with mdadm. If you do the opposite, you will get very difficult to manage the array which is also not very fast. And if any of the disks fail, it will be very difficult to replace them. And if the use of RAID arrays allows to somewhat increase the level of data safety, different kinds of add-ons, such as LVM, can cause the loss of important data instead of adding new functionality.
In such a situation one should immediately use a professional RAID reconstructor which will be able to provide successful data recovery of the RAID array. Launch the application after installing. RS RAID Retrieve offers three options to choose from: Automatic mode — allows you to simply specify the drives that made up the array, and the program will automatically determine their order, array type, and other parameters; Search by manufacturer — this option should be chosen if you know the manufacturer of your RAID controller.
This option is also automatic and does not require any knowledge about the RAID array structure. It will start the process of detecting the array configurations. Step 4: After the constructor builds the array — it will appear as a regular drive.
Double left-click on it. The File Recovery Wizard will open in front of you.
Difference between lvm and mdadm replace btc japan price
How to replace a Disk in LVM in RHEL7 - Linux - RHEL7 - LVM
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