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some inquiries about external ext4 journal[solved]
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Adel Ahmed
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PostPosted: Thu Jul 10, 2014 9:16 am    Post subject: some inquiries about external ext4 journal[solved] Reply with quote

I have 2 gentoo machines:
laptop: 240GB ssd + built in card reader + 500 GB hdd(mechanical)
PC: 2 500GB hdd(mechanical) raid 0 config + tons of usb ports + 14GB of ram

I'm wondering about the performance benefits gained if you configure an external journal, all my current file systems are ext4

laptop:
I plan to use the card reader to install a 2 GB microsd and use it as the external journal device
a. will that give me any performance enhancement for the ssd, if I move the journal writes off the ssd, will that increase its lifespan by a moderately signifcant period?
b. will it give any performance benefit(significant or moderately significant)
c. if the answer to b is yes, what mount option should I use in this case for the best performance(data=?, journal=?)

PC:
I'm thinking of installing a cheap kingston 8GB usb card(for the low latency and better than hdd iops, it's slow though)
a. performance benefit for 2 hdds configured in raid?
b. I could cough up $50 for a 60GB ssd (I live in a 3rd world country with a poor economy, what's cheap for you is slightly expensive for me), this will cost me a new multi-sata port power supply and the SSD itself, I'll definitely install gentoo on the ssd If I get one though
c. I have esata and usb3 ports should I go for an external ssd instead(I don't want to bottleneck the ssd)
d. cheap alternatives?

also any io performance enhancements in general?
I'm willing to try out new file systems if they prove to be faster and better in performance. performance is what's important to me, my data is of little value, I also don't mind countless hours of configuration as long as things will be faster afterwards, I can always format and recopy everything so flexibility is not so critical.


thank you for taking the time to read all of this and thanks in advance for any advice.


Last edited by Adel Ahmed on Sun Jul 13, 2014 12:29 pm; edited 1 time in total
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Adel Ahmed
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PostPosted: Fri Jul 11, 2014 9:12 am    Post subject: Reply with quote

any help/tips?
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Hu
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PostPosted: Sat Jul 12, 2014 12:02 am    Post subject: Reply with quote

I avoided your post because I wanted to leave it unanswered in case someone more knowledgeable might find it that way. Now that your post has been answered by your own bump, I will offer what I can.

The journal is used often enough that you should try to place it on a device at least as fast as the device which holds the journaled filesystem. In most cases, that suggests not putting the journal on USB-attached flash. You will get a small lifespan boost from reducing writes to the SSD, but based on my readings about current SSDs from major manufacturers, lifespan is already good enough that you are unlikely to wear an SSD to death with anything short of a near constant write workload. The answer also depends on how long you plan to keep the SSD. I am happy to use a modern SSD as my primary system storage on a Gentoo system that sees typical updates and moderate usage as a desktop system.
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Adel Ahmed
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PostPosted: Sun Jul 13, 2014 8:10 am    Post subject: Reply with quote

thank you for your answer, so there's no point in moving the journal off the ssd, what about the rotational hard disks?
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NeddySeagoon
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PostPosted: Sun Jul 13, 2014 9:24 am    Post subject: Reply with quote

blakdeath,

Go carefully with external journals. They must be available to mount the filesystem that they apply to, so it won't work for root.
As Hu says, the journal device needs to be at least at fast as the filesystem its journalling for.

For rotating rust, you will save some head movements on writes, so there is no speed improvement for filesystems that are mostly read only, like /usr
FLASH cards may not be a good choice for the journal either, unless you know that they support trim.

All in all, the extra complexity is not worth the marginal gains. Thats for the default ext4 journal setting of metadata only.
If you want to journal the written data too, it may be different.
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those that do backups
those that have never had a hard drive fail.
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Adel Ahmed
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PostPosted: Sun Jul 13, 2014 12:29 pm    Post subject: Reply with quote

thanks alot Neddy, that's very helpful, I'm gonna remove the journals on my laptop since it's battery backed and I never encounter any crashes or panics, I'll leave the PC be.
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