Power Failure

[Vishesh Gupta]

We have power outages, and I am not using a UPS for my monstrous 1000w Server. The power outages are not frequent, could be once a day, or once a month, I use my server for 4 or 5 hours a day, and is not up 24x7. I am not using a parity drive for my array. In case the server shuts off due to power loss, is there any possibility for data loss at idle / read? I am using WD Gold Enterprise drives

[pwm]

37 minutes ago, Vishesh Gupta said:

In case the server shuts off due to power loss, is there any possibility for data loss at idle / read?

 

Often not - but no guarantee.

 

There are many steps where things can go wrong.

 

For the individual file systems, the file system developers tries to make use of barriers and journaling so the file system can roll back to a previous state if only some parts of the writes did reach the disk when the power was lost.

 

Better hard disks tries to make use of stored rotational energy to finish any pending writes if the power is lost. But no disk manufacturer is interested in posting any scientific paper giving the probabilities that the drive may goof if performing writes when the power is lost.

 

With unRAID you have one more factor - unRAID needs to write changes to multiple disks to keep the parity drive(s) up to date with changes to data disks. It's impossible to force concurrent writes to multiple disks and get all disks to either accept or reject the write in case of a power loss. So you will normally get a number of differences in the parity data after a power loss - how many differences will depend on how much writes the machine was busy doing.

 

Normally, unRAID wants to correct the parity after you turn it on again after an unclean shutdown. But if you happen to then find  one or more disks broken so you really need to depend on the parity to recreate a data disk, then these blocks of incorrect parity data can result in a number of random data being written to the recreated data disk - and you will not know where they happen to end up. So you might get broken data files. But you might also get a broken file system in a way that the file system developers didn't expect to happen.

 

So you should definitely consider getting a UPS for your server if you have as many power outages as you seem to indicate. You are playing Russian roulette without knowing how many chambers and how many bullets - one of the power failures just might result in a larger data loss. Another thing is that the actual mains voltage is often very unreliable around a power failure - there may be brownout situations and power spikes. And this can break the computer hardware or make the machine issue incorrect writes to the disks. If you fry the PSU, it could burn every single disk in the machine for a perfect 100% data loss.

 

 

[Vishesh Gupta]

I would like to reiterate, I am not using any Parity, further, I would like to know is there any possibility of any already stored data loss if no writes are being done?

[pwm]

3 minutes ago, Vishesh Gupta said:

I would like to reiterate, I am not using any Parity, further, I would like to know is there any possibility of any already stored data loss if no writes are being done?

 

The probability is low - but as I mentioned you could suffer from data overwrites caused by brownouts or power surges could fry everything.

 

A good UPS do way more than supply power until the computer can safely shutdown. It also takes care of overvoltages and undervoltages and filters power surges. You could get a surge arrestor to protect from surges but only a good UPS will make a good job handling over- or under-voltages.

[Frank1940]

It also depends a lot of the PS that you have in your system.  Many PS will work with any voltage between (say)   85V and 250V while others are only rated for more restricted ranges (say) 105V  to 140V.  The wide range PS's do a much better of providing staple output voltages than ones with more restricted input voltage specs. Of course, no PS will supply voltage within specs when the power cuts out for more than a couple of seconds (max)!  And many, a lot less. 

 

Unfortunately, no one can predict what will happen in any given situation when a power failure happens.   That is why it is recommended that your server have a UPS.   

[ken-ji]

3 hours ago, Vishesh Gupta said:

I would like to reiterate, I am not using any Parity, further, I would like to know is there any possibility of any already stored data loss if no writes are being done?

IF and only IF all the disk IO/pending writes to both data and journal have been completed and flushed from cache, you won't loose any data at all.

And as stated by other people, power outages can come with voltage dips and surges and can cause other weird out of spec power effects - the drive suddenly trying to write garbage; the drive head lands immediately (I think modern hardware prevents this) - these can cause actual lost of data more than the power simply cutting out (yanked power cord, blown fuse, etc)

[JonathanM]

6 hours ago, Vishesh Gupta said:

We have power outages, and I am not using a UPS

Was that a simple statement of current condition, or a declarative statement about the future?

 

If you do not intend to ever run a UPS, then I would tell you to expect data corruption at some point in the future, regardless of OS.

[Vishesh Gupta]

Thank you all for your input. I was not aware that there can be overwrites caused by brownouts damaging the stored data. Would a regular Inverter UPS that is generally used for home use be ok?  I think they are usually offline UPS. Or do I Need to have an online UPS?

[JonathanM]

2 minutes ago, Vishesh Gupta said:

Thank you all for your input. I was not aware that there can be overwrites caused by brownouts damaging the stored data. Would a regular Inverter UPS that is generally used for home use be ok?  I think they are usually offline UPS. Or do I Need to have an online UPS?

Depends on the power supply. Some PSU's are more tolerant than others, typically a higher quality PSU will do more filtering and conditioning to ensure smooth DC output even when the AC input is a little rough. So, a cheap PSU running on the ragged edge of capacity, better have an online high quality UPS. A high quality PSU running in the sweet spot for efficiency can get by with much worse AC coming out of the UPS.

 

(are there enough acronyms in there? I started to get a little acronym punch drunk)

[Vishesh Gupta]

31 minutes ago, jonathanm said:

Depends on the power supply. Some PSU's are more tolerant than others, typically a higher quality PSU will do more filtering and conditioning to ensure smooth DC output even when the AC input is a little rough. So, a cheap PSU running on the ragged edge of capacity, better have an online high quality UPS. A high quality PSU running in the sweet spot for efficiency can get by with much worse AC coming out of the UPS.

 

(are there enough acronyms in there? I started to get a little acronym punch drunk)

 

https://www.amazon.in/APC-UPS-Model-BR1000G-Battery/dp/B0038ZTZ3W/ref=sr_1_2?ie=UTF8&qid=1533056648&sr=8-2&keywords=apc+smart+ups

 

Could you please have a look at the above link for the UPS. Would this be ok? 

 

Server Link:

 

https://www.ebay.com/itm/4U-Supermicro-24-Bay-6Gbs-UNRAID-FREENAS-Storage-server-2x-Xeon-Sandy-Bridge-CPU/132715478149?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

[Frank1940]

That rack is equipped with a 950W PSU.  The CPU is rated at 85 watts each.  The peak starting currents for the 24 HD's would be approximately 576W (24 drives X 2 Amperes (peak)/drive X 12 Volts).  That would give a rough estimate of 750W for the rack.  The BR-1000 is only rated at 600W.  (There is a difference between Volt-amperes and Watts!)  You should probably be looking the BR-1500 unit unless you have substantially less than 24 HD's in your system.  

[Vishesh Gupta]

Thanks a lot. You guys are awesome. I know that the discussion went beyond the scope of UnRaid, but you guys went above and beyond to help me out. Keep up with the great work. Really appreciate it.