1 files changed, 43 insertions, 3 deletions
diff --git a/unit3/unit3.typ b/unit3/unit3.typ
index 531d8ea..e85107f 100644
--- a/unit3/unit3.typ
+++ b/unit3/unit3.typ
@@ -138,7 +138,7 @@ _RAID is a storage technology in which data is written in blocks across multiple
 === Types of RAID
 ==== Striping
 #figure(
-  image("./images/striping.png")
+  image("./assets/striping.png")
 )
 _Striping is a technique to spread data across multiple drives in order to use drives in parallel and increase performance as compared to the use of a single drive._
 - Each drive as a predefined numbwe of contiguously addressable blocks called a strip.
@@ -147,7 +147,7 @@ _Striping is a technique to spread data across multiple drives in order to use d
 - Does not provide any data protection.
 ==== Mirroring
 #figure(
-  image("./images/mirroring.png")
+  image("./assets/mirroring.png")
 )
 _Mirroring is a technique in which the same data is stored simultainously in two different drives, resulting in two copies of data. This is called a "Mirrored Pair"._
 - Even if one fails, the data is safe in the surviving drive.
@@ -163,4 +163,44 @@ _Mirroring is a technique in which the same data is stored simultainously in two
 _Parity is a RAID technique to protect striped data from drive failure by performing a mathematical operation on individual strips and storing the result on a portion of the RAID group._
 - RAID controller finds parity using techniques like XOR.
 - Parity data can be stored on seperate drives or distributed across drives in a RAID group.
-- Parity is calculated everytime data is modified.
+- Parity is calculated everytime data is modified, affecting the performance.
+=== RAID levels
+#table(
+  columns: (auto, auto),
+  table.header([ RAID Level ], [ Meaning ]),
+  [ RAID 0 ], [ Striped set with no fault tolerance. ],
+  [ RAID 1 ], [ Disk Mirroring ],
+  [ RAID 1+0 ], [ Nested RAID ( striping and mirroring ). ],
+  [ RAID 3 ], [ Striped set with parallel access and a dedicated parity disk. ],
+  [ RAID 5 ], [ Striped set with independent disk access and distributed parity. ],
+  [ RAID 6 ], [ Striped set with independent disk access and dual distributed parity. ]
+)
+== Data Access methods
+- External storage can be connected directly or over network.
+- Applications request data by specifying file name and location.
+- File systems map file attributes to logical block address (LBA).
+- LBA simplifies addressing by using a linear address to access a block of data.
+- File system converts LBA to a physical/cylinder-head-sector/CHS address and fetches data.
+=== Three schemes of data access.
+#figure(
+  image("./assets/dataaccessmethods.png")
+)
+==== Block Level Access
+- Storage volume is created and assigned to the compute system.
+- Application data request is sent to file system and converted to block-level request. 
+- Request sent to the storage system.
+- Converts LBA to CHSA and fetches data in block sized units.
+==== File Level Access
+- File system created on a seperatee file server.
+- File-level request sent to file server.
+- File server converts file-level request to block-level request.
+- Then block-level request is sent to storage.
+==== Object Level Access
+- Data is accessed over the network in terms of self contained objects.
+- Each object has a unique object identifier.
+- Application request is sent to file system.
+- File system communicates with the object-based storage device (OSD) interface.
+- OSD interface sends the request to the storage system.
+- Storage system has OSD storage component.
+- This component manages access to the object on the storage system.
+- OSD storage component converts object-level request to block-level request.