J
John
Dear All,
I've got a question regarding hardware RAID 1 (R1) and RAID 5 (R5)
Don't consider following issues, because they are not playing in this
question:
a) Minimum disk setup and setting up hot spares
b) Write intensive situations or sequential reads situations
c) Dynamic resizing a RAID
d) Same manufacturer and same lot drive issues (this count for both setups)
e) Performance issue when 1 HD goes down.
Look at following diagram for better understanding :
RAID 1 :
--------
HD1 HD2 HD3 HD4
------------------------------------------------
Strip1 Strip2 Copy1 Copy2
Strip3 Strip4 Copy3 Copy4
Strip5 Strip6 Copy5 Copy6
Strip7 Strip8 Copy7 Copy8
HD1 & HD2 are duplicated on HD3 & HD4. If HD1 crashes then the copy is
simply used instead. Recovery is simply; replace HD1 and copy the entire HD3
to it.
RAID 5 :
--------
HD1 HD2 HD3 HD4
------------------------------------------------
Strip1 Strip2 Strip3 Parity1-3
Strip4 Strip5 Parity4-6 Strip6
Strip7 Parity7-9 Strip8 Strip9
Parity10-12 Strip10 Strip11 Strip12
Instead of real duplicating disks like R1, R5 is creating a parity (XOR) and
distributes this round robin wise on all disks.
Question :
----------
I feel more secure when data is written to a R5 then it is on a R1. Why?
scenario RAID 1 : Suppose that strip 1 is written on HD1 and duplicated on
HD3, but there was a bad sector on HD3, so a real sync copy would never
work. When HD1 fails after 1 year, and it's replaced and I restore a copy of
HD3 on it, then my guess is that "Original HD1" and "restored HD1" are never
identical or you have to mark bad sectors that came from HD3 to the new
restored HD1 and still then there is a difference with "Original HD1" and
"restored HD1".
scenario RAID 5 : All this will never happen because there is no identical c
opy of data. If a sector is going bad on HD1 then this will be marked and
data will be written on an other sector on HD1. When HD1 is failing then
removing and inserting a new one will generate automatic new data recovered
from HD2, HD3 & HD4. This is very strait forward.
Why this question :
-------------------
I feel that RAID 1 was intended for fast realtime backup, and when HD1 is
giving huge problems, you can boot from the backup HD. I don't feel that
this system was made for "keeping data online without a second of
interruption"
If feel that RAID 5 was made for "keeping data online without a second of
interruption" and must be seen in that way. So RAID 5 could be seen as a
successor of RAID 1. (Please do not use points a,b,c,d & e as the BUT story)
Also, a hard-disk can go bad in a heartbeat, but can also slowly give some
hints that there is something wrong (sectors going bad on a certain place).
And most of the time a slowly death is what he will do. Am I right that RAID
1 will not give a solution for slowly death but RAID 5 will.
Am I right? Please do not take a,b,c,d & e points into consideration because
they are not the basics for this questions. I want to go to the basics of
RAID 1 and RAID 5 in online system interruption?
I can go 1 step further and say that RAID 0 was a great solution for gaining
bandwidth and with no much effort a backup system could be made and they
named it RAID 1. But no much thinking was done for the backup solution if
you take online data in account that cannot be interrupted.
If I read some articles on RAID 1 then I read a lot of "Then you can restart
from the backup disk". And this is what start me thinking and did a lot of
research on it.
Please, do give me your opinion.
Kind regards,
John.
I've got a question regarding hardware RAID 1 (R1) and RAID 5 (R5)
Don't consider following issues, because they are not playing in this
question:
a) Minimum disk setup and setting up hot spares
b) Write intensive situations or sequential reads situations
c) Dynamic resizing a RAID
d) Same manufacturer and same lot drive issues (this count for both setups)
e) Performance issue when 1 HD goes down.
Look at following diagram for better understanding :
RAID 1 :
--------
HD1 HD2 HD3 HD4
------------------------------------------------
Strip1 Strip2 Copy1 Copy2
Strip3 Strip4 Copy3 Copy4
Strip5 Strip6 Copy5 Copy6
Strip7 Strip8 Copy7 Copy8
HD1 & HD2 are duplicated on HD3 & HD4. If HD1 crashes then the copy is
simply used instead. Recovery is simply; replace HD1 and copy the entire HD3
to it.
RAID 5 :
--------
HD1 HD2 HD3 HD4
------------------------------------------------
Strip1 Strip2 Strip3 Parity1-3
Strip4 Strip5 Parity4-6 Strip6
Strip7 Parity7-9 Strip8 Strip9
Parity10-12 Strip10 Strip11 Strip12
Instead of real duplicating disks like R1, R5 is creating a parity (XOR) and
distributes this round robin wise on all disks.
Question :
----------
I feel more secure when data is written to a R5 then it is on a R1. Why?
scenario RAID 1 : Suppose that strip 1 is written on HD1 and duplicated on
HD3, but there was a bad sector on HD3, so a real sync copy would never
work. When HD1 fails after 1 year, and it's replaced and I restore a copy of
HD3 on it, then my guess is that "Original HD1" and "restored HD1" are never
identical or you have to mark bad sectors that came from HD3 to the new
restored HD1 and still then there is a difference with "Original HD1" and
"restored HD1".
scenario RAID 5 : All this will never happen because there is no identical c
opy of data. If a sector is going bad on HD1 then this will be marked and
data will be written on an other sector on HD1. When HD1 is failing then
removing and inserting a new one will generate automatic new data recovered
from HD2, HD3 & HD4. This is very strait forward.
Why this question :
-------------------
I feel that RAID 1 was intended for fast realtime backup, and when HD1 is
giving huge problems, you can boot from the backup HD. I don't feel that
this system was made for "keeping data online without a second of
interruption"
If feel that RAID 5 was made for "keeping data online without a second of
interruption" and must be seen in that way. So RAID 5 could be seen as a
successor of RAID 1. (Please do not use points a,b,c,d & e as the BUT story)
Also, a hard-disk can go bad in a heartbeat, but can also slowly give some
hints that there is something wrong (sectors going bad on a certain place).
And most of the time a slowly death is what he will do. Am I right that RAID
1 will not give a solution for slowly death but RAID 5 will.
Am I right? Please do not take a,b,c,d & e points into consideration because
they are not the basics for this questions. I want to go to the basics of
RAID 1 and RAID 5 in online system interruption?
I can go 1 step further and say that RAID 0 was a great solution for gaining
bandwidth and with no much effort a backup system could be made and they
named it RAID 1. But no much thinking was done for the backup solution if
you take online data in account that cannot be interrupted.
If I read some articles on RAID 1 then I read a lot of "Then you can restart
from the backup disk". And this is what start me thinking and did a lot of
research on it.
Please, do give me your opinion.
Kind regards,
John.